Kill all formless abomination spawns without killing any of the dragonmarked ritualists. To do so, range pull the oozes back into the tunnel. This will also aggro nearby erudites, however they have a shorter leash than the oozes. If you pull and run to about ( -22, -53, 24 ) Copy/waypoint -22, -53, 24, any aggroed erudites will have leashed back, allowing you to kill the oozes with ease. Once all the oozes have been killed, a message will appear in your chatbox saying "A supernatural chill invades in the chamber". This is the signal that the event has started. Now and until Ishi spawns, the erudites will routinely chant and you will get updates in your chatbox.
Kill all formed abominations while still not killing the dragonmarked ritualists. Ishi will spawn in a cloud of green mist shortly after the death of the final abomination. One the mist spawns, it's safe to kill the erudites. WARNING:The Erudites themselves will actually attempt to engage the final abomination themselves once it spawns, 'tagging' it as an invalid encounter. If you kill Ishi before you remove the ritualists from its hate-list, you will lose the opportunity for the kill and its loot (the same as any other encounter in-game that is already engaged with NPCs)