The Litany of Adralla is a tool you'll receive after defeating the Sloth demon in the Fade, which you can use to combat Uldred.
In order to use it in battle, open the radial menu and select advanced. It is in there at the bottom of the advanced radial menu. Use it when Uldred causes pink circles to surround a mage. Be warned he does this at certain points as his health gets lower and the Litany does have a cooldown. If used in time, you'll stop a mage from becoming an abomination. If not, you'll do heavy damage to the new abomination.
If you are unable to see the pink circles because of combat, you will hear Wynn call out for you to use it now.