Z'rell is a high-ranking member of the Cult of the Absolute and a trusted advisor to Ketheric Thorm.
Involvement
Z'rell is first encountered in the Moonrise audience chamber along with General Thorm, where they are judging the failures of the Absolute's forces from the Goblin Camp. Ultimately, Z'rell delegates their punishment to the player, and orders them to report to her afterwards. There, she assigns them the task of assisting the necromancer Balthazar in securing a relic needed by Ketheric, called the Nightsong. If the player allows Balthazar to take the Nightsong Z'rell will give a follow-up quest, Resolve the Abduction, tasking the player to assist Marcus with the abduction of Isobel.
Unlocking Lann Tarv's secret stash
The trader Lann Tarv has a secret stash of items that can be unlocked by convincing Z'rell to give you access to it.
If the dialogue option "What exactly was this relic he was sent to retrieve?" doesn't appear, cast Detect Thoughts. After choosing it, you will need to pass an insight check followed by a DC 18 persuasion check. If you succeed, speak with Lann Tarv, with the same character that convinced Z'rell, in order to unlock the secret stash.
Assault on Moonrise Towers
If Z'rell is still alive after the player kills or frees the Nightsong, she will lead the forces of the Absolute against the party when they attempt to enter Moonrise Towers. She will start this battle with Mirror Image and Fire Shield: Chill.
If it is higher level than the spell slot you used to Counterspell, you must make a check using your spellcasting ability[See Notes] to prevent it. The check's difficulty is equal to 10 plus the level of the spell you are trying to counter.