I can't kill demonuminon!?
When Demonuminon reaches about 90% damage he will issue a 'just you and me' challenge, but can still be attacked.
However, when injured to about 95% he will drop all 3 orbs (chaos, neutral and lawful)
and can no longer be attacked. Only the follow icon will be seen.
This is NOT a bug.
Get Aurax to pick up all 3 of these tiny objects and Demonuminon will once again become a valid attack target.
(Note that Vex's cut scene stops the third orb from being picked up, and the pickup command will need to be reissued).
A couple more bolts of lightning or some such weapon and Demonuminon will be no more, and the crowds will cheer.