I saw one Voidsoul Darachnid but a pilgrim killed it before I could get in the fight. You will take a hefty faction hit killing Drixies and Pilgrims. How do you tell which are Voidsouls before you engage?
I think that because of the differing factions in Firionia Vie the NPCs will fight each other and The Possessed Voidsoul becomes unvieled before killing the NPC it is fighting, then goes on it's merry way still unvieled. My The Possessed Voidsoul was in the form of a Firionia Vie Drixie and I took Faction hits as if it were. So, if you're concerned about what faction hits you're getting pay attention to the form of THE Possessed Voidsoul you decide to kill.
seem to take on the form of pilgrims (saw one as a male high elf, another as a male dwarf) and are named The Possessed Voidsoul (for those with track). they seem to walk between the succor point, and south of the Leech Field.