Right click and place staves in each of the five rocks - one of which is above the ledge
When all are placed, Altairenanhk will come flying. If he doesn't appear straightaway then zone out and in again. The actual update is from the kill, not the staves, so you can do this in a group. Altairenanhk has a knockback, so either fight in the cave, or pull back to land. If you die in the attempt, you can resummon without having to redo any steps.