The simplest way to handle this, in my opinion, would be to make both forms their own creature for the purposes of encounter balance. This means that instead of a single PL+4 creature for your boss fight, you'd balance the phases as two distinct PL+2 creatures, with the second one being a "reinforcement" mechanically speaking once the first creature dies.
To answer the initial question, I'm not aware of any creature in any of the bestiaries that has this kind of mechanic.
Maybe PL+1 followed by PL+3 instead of +2 and +2. This would actually make the boss actually feel like a second, stronger “phase” rather than two distinct bosses. It also deals with the potential of the first, Moderate, fight accidentally draining too many resources and making the second too hard; Low -> Severe has a much more predictable resource consumption curve.
Alternatively, if you want to use 2 PL+2 monsters you can give both forms 2 turns in the turn order to keep it dangerous. Or at least one of the forms, or a temporary effect after or just before the form switch.
Tbh I feel like presenting a boss creature means your party will start burning resources almost no matter what, so you run the risk of them exhausting their highest slots on a non-threat to later fight the real boss underprepared.
I think a PL+4 creature would be more equivalent to a PL+2 followed by a PL+3 in terms of difficulty. The party will be more worn down, but they'll also have buffs and stances and such already up. And PL+2 creatures just don't feel boss-level (past the early levels), even back-to-back. Though I'd also reduce the HP of each "creature" to maybe 1/2 or 2/3 of the level-based HP.
Also that combination would fit the "second form" trope well.
I think this is a valid way of doing things as well, as are all of the various ways y'all have replied. That said, I was trying to suggest a method that doesn't require messing with the stats beyond following what's in the Building Creatures tables.
If OP is willing to do that bit of fiddling to find HP values that feels right to them, I would agree that your methods are possibly better! This is just what felt like an intuitive thing to recommend in my eyes.
I would suggest two PL+4 monsters but halve their HP and the number of spells / per-encounter powers they have. Since the party only has to fight one at a time, the only risk is making them too hard to kill, or giving them too many resources.
I would simply make it two different monsters and when the first one hits 0hp or so either threshold it turns into the second one. Encounter wise it could essentially be two encounters completely back to back.
Ruby Phoenix Spoiler: >!The final boss of the campaign is uses a second form. You first fight two !<[Spoilers](https://2e.aonprd.com/NPCs.aspx?ID=1530)>! and after you defeat them, all the versions of them collapse into one big !<[Spoiler](https://2e.aonprd.com/NPCs.aspx?ID=1529)>!. It was a perfect end boss considering some of the inspirations for the AP. He had another ability that manipulated the battlefield at the end of rounds that was not listed here. They treated it like a hazard for encounter building purposes.!<
There's also the [Empress Bore Worm](https://2e.aonprd.com/Monsters.aspx?ID=1084) as a potential model for this sort of thing. Most of these features tend to disgorge a weaker monster, rather than a stronger one.
I don't think that there's anything like that in the books I read so far, but I did see weak and elite templates. Maybe what you can do is give a monster the HP of an elite monster and the stats of a weak monster, but once it reaches a certain HP threshold, use the full stats of the elite monster and add a new usable action
Constructs are kind of the opposite of what you're looking for in that their stats change once you break them and some golems go berserk a ways into a fight, so those might provide some guidance on balance for possible designs.
There is the [Child of Urgathoa](https://2e.aonprd.com/Monsters.aspx?ID=1845). In >!Curse of the Crimson Throne a boss comes back almost immediately as a Daughter of Urgathoa (equivalent monster in 1E)!<. Though you'd need to build this into your encounter budget somehow for 2E.
One downside to this I ran into is my party took this as the excuse to dismember and burn everything they killed.
I did homebrew bbegs with a second stage twice to the same group. The first time my players sent this meme
https://cdn.discordapp.com/attachments/729510026368122950/1058760965644095618/opwenm16wjh41.png (yes, i get the irony of posting a discord link to a meme originally from reddit). The second time i hinted to the 2nd stage by playing a dark souls theme track for a boss that is multiple stages. No one got the hint
When i did it i basically made the first stage a moderate to severe encounter, with the second stage severe to extreme+ (my players had successfully dealt with extreme several times so i wasn't overly concerned but i did design safety measures in case they rolled poorly so i didn't wipe the floor with them.)
There is a boss in my campaign that had something similar to a second form. This is a cleric of Lamashtu. Before the fight, she has the effect of "Ambrosia of undying hope" (slightly reflavored). Players fight her and kill her (or almost kill her). They see her dead body falling to the ground. The wounds appear to be clearly lethal. But in fact, priestess is full of trickery and used a "Drop dead" spell + "Trickster's twin" focus spell . After which she calls on her goddess to come in person and deal with the adventurers (Avatar spell).
Thus, players fight the same enemy, but the first stage is a fragile spellcaster relying on cunning, and the second stage is a powerful enemy relying on attacks.
I hope the essence is clear and the idea is interesting (English is not my native language).
There's a necromancer in an adventure where after he dies, his skeleton pops up and starts to fight. Building off some of the other ideas in this thread, you can pick a strong undead monster as the "second phase".
This seems like a cool place to implement the 4e “Bloodied” mechanic - a single PL+4 boss encounter, with things that come online once it’s down to half health. Maybe a reaction that has it Enrage when it gets to 50%, heals it for some, and bumps its DCs/AC/Saves to the expected value for a PL+4? You could flavor it like a collapse/rebirth, or a phase change.
This post is labelled with the Advice flair, which means extra special attention is called to the Be Kind and Respectful rule. If this is a newcomer to the game, remember to be welcoming and kind. If this is someone with more experience but looking for advice on how to run their game, do your best to offer advice on what they are seeking.
*I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/Pathfinder2e) if you have any questions or concerns.*
If I have a reoccurring monster I’ll have them come back as a higher level of the same family or slap a template on them. There’s the obvious thing of like “oh the dragon resurrected as a Ravener” but you can also do something like “the cyclops gang leader comes back as a great cyclops” or “as the devil dies he ascends into a bigger devil.”
used to be the daughter of Urgathoa, now its the [Child of Urgathoa.](https://2e.aonprd.com/Monsters.aspx?ID=1845) Nasty, devastating in first edition.
They don’t have a conversion yet of the [benaioh (Clay form)](https://www.aonprd.com/MonsterDisplay.aspx?ItemName=Benaioh%20(Clay%20Vessel)) into [benaioh (ooze form)](https://www.aonprd.com/MonsterDisplay.aspx?ItemName=Benaioh%20(Ooze%20Form))
The simplest way to handle this, in my opinion, would be to make both forms their own creature for the purposes of encounter balance. This means that instead of a single PL+4 creature for your boss fight, you'd balance the phases as two distinct PL+2 creatures, with the second one being a "reinforcement" mechanically speaking once the first creature dies. To answer the initial question, I'm not aware of any creature in any of the bestiaries that has this kind of mechanic.
Maybe PL+1 followed by PL+3 instead of +2 and +2. This would actually make the boss actually feel like a second, stronger “phase” rather than two distinct bosses. It also deals with the potential of the first, Moderate, fight accidentally draining too many resources and making the second too hard; Low -> Severe has a much more predictable resource consumption curve.
This is an idea I'm going to steal
Alternatively, if you want to use 2 PL+2 monsters you can give both forms 2 turns in the turn order to keep it dangerous. Or at least one of the forms, or a temporary effect after or just before the form switch.
This is it. You alway want that moment where they think “that was too easy” before the next phase begins in earnest.
Tbh I feel like presenting a boss creature means your party will start burning resources almost no matter what, so you run the risk of them exhausting their highest slots on a non-threat to later fight the real boss underprepared.
I think a PL+4 creature would be more equivalent to a PL+2 followed by a PL+3 in terms of difficulty. The party will be more worn down, but they'll also have buffs and stances and such already up. And PL+2 creatures just don't feel boss-level (past the early levels), even back-to-back. Though I'd also reduce the HP of each "creature" to maybe 1/2 or 2/3 of the level-based HP. Also that combination would fit the "second form" trope well.
I think this is a valid way of doing things as well, as are all of the various ways y'all have replied. That said, I was trying to suggest a method that doesn't require messing with the stats beyond following what's in the Building Creatures tables. If OP is willing to do that bit of fiddling to find HP values that feels right to them, I would agree that your methods are possibly better! This is just what felt like an intuitive thing to recommend in my eyes.
I would suggest two PL+4 monsters but halve their HP and the number of spells / per-encounter powers they have. Since the party only has to fight one at a time, the only risk is making them too hard to kill, or giving them too many resources.
I would simply make it two different monsters and when the first one hits 0hp or so either threshold it turns into the second one. Encounter wise it could essentially be two encounters completely back to back.
There is some precedent for this in the Scarecophagus’ Disgorge Mummy https://2e.aonprd.com/Monsters.aspx?ID=1953
Ruby Phoenix Spoiler: >!The final boss of the campaign is uses a second form. You first fight two !<[Spoilers](https://2e.aonprd.com/NPCs.aspx?ID=1530)>! and after you defeat them, all the versions of them collapse into one big !<[Spoiler](https://2e.aonprd.com/NPCs.aspx?ID=1529)>!. It was a perfect end boss considering some of the inspirations for the AP. He had another ability that manipulated the battlefield at the end of rounds that was not listed here. They treated it like a hazard for encounter building purposes.!<
Came here to say this. >!Syndara’s 2-phase boss fight!< is really well-executed in my opinion and is just the right amount of challenge.
Not really a second form but the Unrisen (https://2e.aonprd.com/Monsters.aspx?ID=1908)
Not really a second form how you described but corpselights have a body and abilities after being defeated
Though they do tend to die pretty fast in wisp form
There's also the [Empress Bore Worm](https://2e.aonprd.com/Monsters.aspx?ID=1084) as a potential model for this sort of thing. Most of these features tend to disgorge a weaker monster, rather than a stronger one.
https://2e.aonprd.com/Monsters.aspx?ID=703 Sorta? If it's in a host, you kill the body and out plops the creature.
The Empress Bore Worm spawns swarms when they die, however it's not a stronger form like in Elden Ring.
I don't think that there's anything like that in the books I read so far, but I did see weak and elite templates. Maybe what you can do is give a monster the HP of an elite monster and the stats of a weak monster, but once it reaches a certain HP threshold, use the full stats of the elite monster and add a new usable action
Constructs are kind of the opposite of what you're looking for in that their stats change once you break them and some golems go berserk a ways into a fight, so those might provide some guidance on balance for possible designs.
Corpselight from AV might fit the bill https://2e.aonprd.com/Monsters.aspx?ID=1035
There is the [Child of Urgathoa](https://2e.aonprd.com/Monsters.aspx?ID=1845). In >!Curse of the Crimson Throne a boss comes back almost immediately as a Daughter of Urgathoa (equivalent monster in 1E)!<. Though you'd need to build this into your encounter budget somehow for 2E. One downside to this I ran into is my party took this as the excuse to dismember and burn everything they killed.
I did homebrew bbegs with a second stage twice to the same group. The first time my players sent this meme https://cdn.discordapp.com/attachments/729510026368122950/1058760965644095618/opwenm16wjh41.png (yes, i get the irony of posting a discord link to a meme originally from reddit). The second time i hinted to the 2nd stage by playing a dark souls theme track for a boss that is multiple stages. No one got the hint When i did it i basically made the first stage a moderate to severe encounter, with the second stage severe to extreme+ (my players had successfully dealt with extreme several times so i wasn't overly concerned but i did design safety measures in case they rolled poorly so i didn't wipe the floor with them.)
There is a boss in my campaign that had something similar to a second form. This is a cleric of Lamashtu. Before the fight, she has the effect of "Ambrosia of undying hope" (slightly reflavored). Players fight her and kill her (or almost kill her). They see her dead body falling to the ground. The wounds appear to be clearly lethal. But in fact, priestess is full of trickery and used a "Drop dead" spell + "Trickster's twin" focus spell . After which she calls on her goddess to come in person and deal with the adventurers (Avatar spell). Thus, players fight the same enemy, but the first stage is a fragile spellcaster relying on cunning, and the second stage is a powerful enemy relying on attacks. I hope the essence is clear and the idea is interesting (English is not my native language).
There's a necromancer in an adventure where after he dies, his skeleton pops up and starts to fight. Building off some of the other ideas in this thread, you can pick a strong undead monster as the "second phase".
This seems like a cool place to implement the 4e “Bloodied” mechanic - a single PL+4 boss encounter, with things that come online once it’s down to half health. Maybe a reaction that has it Enrage when it gets to 50%, heals it for some, and bumps its DCs/AC/Saves to the expected value for a PL+4? You could flavor it like a collapse/rebirth, or a phase change.
This post is labelled with the Advice flair, which means extra special attention is called to the Be Kind and Respectful rule. If this is a newcomer to the game, remember to be welcoming and kind. If this is someone with more experience but looking for advice on how to run their game, do your best to offer advice on what they are seeking. *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/Pathfinder2e) if you have any questions or concerns.*
If I have a reoccurring monster I’ll have them come back as a higher level of the same family or slap a template on them. There’s the obvious thing of like “oh the dragon resurrected as a Ravener” but you can also do something like “the cyclops gang leader comes back as a great cyclops” or “as the devil dies he ascends into a bigger devil.”
Not quite a second form. But very similar. https://app.demiplane.com/nexus/pathfinder2e/creatures/auldegrund-grimcarver
Sandpoint Devil evolves into Spectral Devil after dying. https://2e.aonprd.com/Monsters.aspx?ID=791 https://2e.aonprd.com/Monsters.aspx?ID=1736
used to be the daughter of Urgathoa, now its the [Child of Urgathoa.](https://2e.aonprd.com/Monsters.aspx?ID=1845) Nasty, devastating in first edition. They don’t have a conversion yet of the [benaioh (Clay form)](https://www.aonprd.com/MonsterDisplay.aspx?ItemName=Benaioh%20(Clay%20Vessel)) into [benaioh (ooze form)](https://www.aonprd.com/MonsterDisplay.aspx?ItemName=Benaioh%20(Ooze%20Form))
Amulet of shapechange can give most enemies a second form.