You get this Quest from the White Witch Icara, after you freed her in the Find the Witch! Quest. The normal way to complete this Quest would be to enter the Temple of the Dead, dig up Cassandras body, burn it and return to Cassandra and kill her. You don’t need to complete this Quest, but it makes the later a bit easier.
From the Titan Head (if you have the Titan Dictionary) or the seven Forest Victim’s you learn that Cassandra ‘s Body is buried in the Temple of the Dead/Cassandra’s Temple and that only the dead may enter.
So take one of your Hero to the Entrance (you can kill the Sentinels with the Skill Death Knight Bane) and kill him while he holds the Teleporter Pyramid and active the Button, use the 2nd Teleporter Pyramid to follow and resurrect him.
Near the wooden Bridge is a Oddly-Shaped Skull that lowers the Bridge and a second one near the Big Stone head. There is nothing behind that Stone Head, don’t search like me for hours, how to get pass it 😉
At x:125 y:166 you find a secret Room with Casandra’s Skeleton and a Blood Stone. Dig her up and use a Firespell/Attack/Arror to burn her Skeleton – now she issn’t protected by the Void Aura anymore and you and kill her. Don’t forget to loot the Key of the Source King from her Skeleton (you can travel back to Desecrated Church in Cyseal and open the purple Lock from the Chest of the Source King).
After you killed Casandra, loot the Soulforge Repair Ritual and the Quest is completed
But you can also kill Arhu (Wicked Young Man Achievement) – witch is for my party nearly impossible (13600 HP, 34 (!) Action Points, 700-1300 DMG and 200 Armor).
But I found a glitch to kill anyway, you just need the Skill Feather Drop. Use one Hero to talk witgh him (so he don’t run away) and move him with a other Hero with the Skill Feather Drop step-by-step to x:130, y:170 and put him there over the Lava and let him die. Then return to Cassandra to complete the Quest (and get this Chivo)