Ir Yût, the Deathsinger
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First Crota Fireteam
Encountering Ir Yût and hearing her Deathsong was the primary reason Toland, the Shattered joined the First Crota Fireteam on their quest for Crota. He had been quite interested in the Song for some time, trying to figure out how she could sing it without dying; he speculated that she made herself part of the song and became death incarnate. He did not hide his real goal from the fireteam members, openly talking about it with the fellow Warlock Eriana-3.
Toland did achiever his goal and met Ir Yût in the depths of the Hellmouth. It is unclear how their "tête-à-Yût" actually went like, but it resulted in Toland being unsung from his biological form and banished, in some way, to the Sea of Screams. Ir Yût is also implied to having killed Eriana-3, as her designate is included on the file preserving the moment of Eriana's death recovered from Jax's remains.
Crota's End and The Taken King
Ir Yût resided in the Oversoul Throne, Crota's throne world, at the time of the Guardians' raid on the Hellmouth. She was slain by the Lightbearers, and armor and weapons were fashioned from the spoils. However, she could be found several months later attending Crota's death ceremony in the mission Last Rites, along with the Daughters of Oryx and few other Deathsingers. This points to her possibly being an Ascendant Hive, as well as still being alive somewhere, as killing her in the Oversoul Throne certainly wouldn't cause her final death.
Unlike all the other raid bosses before her, Ir Yût will not immediately be actively attacking any Guardians. Instead, she will have locked herself inside the room containing Crota's summoning crystal. At this time, all players should be focused on killing enemies, namely Shriekers. Once they are destroyed, she will activate her special ability.
Ir Yût is an especially dangerous wizard, as her special power is the Deathsong, which kills anyone who hears it when it is complete. She will start the song once any Guardians are detected, in which they will have three minutes to kill her before she completes it. There will be a countdown for the last 30 seconds of this song, which is when the barrier will be unlocked, allowing the player to unleash all damage on her as possible.
Age of Triumph added a Challenge Mode for Ir Yût. In order to complete it, the players must kill everything in the area (including the Wizards and Shriekers) and then kill her with an Ascendant Sword. Once the first wave of enemies are killed, a message saying "Ir Yût shrieks in fear" will appear on the bottom left. The fireteam must rush into the open area and kill everything (including Ogres) once more to spawn a Swordbearer. Once it is killed, a Guardian must pick up the sword and rush back to Ir Yût and kill her with one swipe.