Spawned for me in the southwestern corner of the coliseum loop. I cleared everything but the yunjos and hynids starting at the dropdown and moved counter-clockwise. After I cleared the last room the first time, he spawned a few seconds later at roughly 540, 280, -36.
[EDIT: still here trying to finish off hunter achievements, saw Warlord Vokken spawn in the same room again, but at 537 319 -37. I hadn't killed a single mob in the coliseum loop. No one else was in the zone either. It spawned, moved east into the arena proper, killed a slave or two and despawned. Seems as though it either shares a PH slot with mobs on the other side of the zone or it's on some sort of script to spawn and despawn mobs in the coliseum loop, independent of a PH.]
Edited, Aug 12th 2021 12:53pm by Andhrimnir