It's not really clear what to do, especially in the transition to phase 2 so here is a breakdown:
I found it easier to do in dragon riding flight style. Make your way to Dimensius and use your cloak's extra action button to port over to his platform.
Phase 1, DPS Dimensius to 0% which will then transition you to phase 2
Phase 2, you will get knocked up in the air and then settle on a dark orb. If you just stand there wondering, now what?, you will fairly quickly fall and die. But as soon as you get knocked in the air and settle on the orb, you will be able to mount up. When you do that fly through the portal rings (like you are doing the dragon riding races) using your dragon riding skills until you reach a small platform. Kill the mini boss there, then mount up, and fly through a second set of rings to another small platform where you kill a second mini boss. Once that one dies, you will get sucked into a portal and dropped onto a bigger platform where you enter Phase 3
Phase 3, Kill adds and DPS Dimensius to 0% health again
Wait for a brief monologue until a clickable portal appears. Click that which will take you to a cinematic and complete the quest