Adralla of Vyrantium dedicated her life to the study of blood magic—the academic study, rather than the practice. A deeply pious mage, she was renowned in her day for having found a counter to every form of mind control, a defense against dream walkers, and even counter-spells to demonic summons.
Her efforts went unappreciated in her native Tevinter, however. After three different magisters attempted to have her killed, she fled the country, choosing to take refuge in the land of Blessed Andraste's birth. She spent the remainder of her days with the Circle in Ferelden.
The Litany of Adralla disrupts the casting of mind-control spells. Use the Litany whenever a creature tries to dominate another with magic, and it will interrupt the casting. Once the spell is in effect and a character is under a blood mage's power, it is too late. —From Codex entry: The Litany of Adralla
This section contains spoilers for: Dragon Age: Origins.
After The Fade: Lost in Dreams has been completed it can be retrieved from Niall's corpse. It is a vital tool to help defeat Uldred. When he starts to use a spell to bring more Abominations to him, using the Litany will prevent the spell from continuing. If Wynne is in the party she will say things like 'use it now' to alert the Warden that the Litany should be used.
The Litany does not have to be used, and allowing Uldred to turn some mages during the battle will allow you to get more experience points. However, if Irving is controlled and killed, it is impossible to side with the mages during the Broken Circle quest.
Note: The Litany cannot simply be used from the inventory in the console versions of Dragon Age: Origins, even at the appropriate time. It must be used from the advanced tactics section of the tactics wheel (upper right).
When used near abominations it delivers a large amount of damage which kills most of them instantly, and it also does some damage to Uldred. On PC, Dog can click on the Litany in his inventory or on his taskbar and get the use item sound, but it does nothing.
This section contains spoilers for: Dragon Age: Asunder.
Lord Seeker Lambert uses the Litany to dispel an "invisibility" spell placed by Cole upon himself and Rhys when they encounter him in the sewers beneath the White Spire. He confronts Cole and accuses him of being a Demon since the Litany was designed to resist demonic influence over the mind.