Reefraider, you're correct...the spawn point is between the two towers...I walked on up coming form the Southeast and, the moment I pass between the towers, Griniash along with a gang of gnolls popped and aggro'd. Hope this helps!
He also gives this quest, Vicious Assault. You have to find and slay Griniash the Vicious who is supposed to spawn near the two towers at the back entrance from Antonica. I received this quest on 06/26/2006 but have yet to find Grinash.