I engaged a couple of them with EVs, any monster that was fighting the EVs when they faded out began following stealthed Tanivar, additional ones that spawn do not. He had wound up going outside to restealth so he could do quick & dirty loot checks.
[Edit] Cast an EV at another, the trog walked all the way to Tanivar then turned to and fought the EV which had followed it over to Tanivar. If Tanivar stealthed away, the Trog followed him until he stopped moving then once again turned to and fought the following EV.