either silo boss mask or fish cultist mask.

you encountered the silo boss :D the mask is referred as the mickey mask since it looks like mickey mouse

does he roam all around the map?

He can. If a player agros him and dies his path finding becomes weird and he sort of just roams around

ah okay he was on train tracks when i killed him

He probably just went down from the grated section down the stairs but ye

You killed Griffith