It was not Darkfall's first time killing Ultraxion, but only about half of us had killed him before. So there was some learning to be done. Fortunately I had some good advice from DraNgNon, who had killed him before on normal mode.
I'm embarrassed to admit that several of the wipes were my own fault, either from not staying in for Hour of Twilight, or not getting out for Fading Light. However I was having a very hard time making sure that I clicked the button at the right time. I mouse with my right hand, and also hit my Vent talk button with the right hand as well. And doing both of those at the same time to call out the bad stuff was not working for me.
So I remembered the cardinal rule of tanking. The cardinal rule of playing WoW. The rule that underlies all the other stuff you need to do: Keybind! Keybind your abilities, but especially keybind your cooldowns. And if Heroic Will isn't a (fight specific) cooldown, I just don't know what is.
/click ExtraActionButton1
So, keybind that sucker: make a macro and put it on a action bar. Once I did that we got him on the next try.