I think the map shows where it is... I did not find a way to those guys. I searched for a hidden switch to open a passage, but could not find anything. Did anyone come across those guys or was this just a bug spawning mobs where there should be none?
It's a bug, especially around alters of lilith. Sometimes enemies just spawn in the terrain for some unknown reason. And because D4 is truly 3D, them being clipped into the floor prevents them from moving, or from being pathed to. Additionally you can't attack them if your line of sight is broken from them. Sometimes you can get them to respawn and fix themselves, but unless you absolutely need to kill them for some reason it's faster to just leave.
It’s not a bug, they’re “devotees of Lilith” making pilgrimage to the altar
The bug is that they spawn in areas where they shouldn't. They're all literally stuck there. There's several altars that have this issue.