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, Thorm delegates their punishment to the player character, and Z'rell 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 party allows Balthazar to take the Nightsong, then Z'rell gives a follow-up quest, Resolve the Abduction, tasking the party to assist Marcus with the abduction of Isobel.
The trader Lann Tarv has a secret stash of items that can be unlocked by convincing Z'rell to give the party access to it.
If the dialogue option "What exactly was this relic he was sent to retrieve?" does not appear, casting Detect Thoughts is recommended. After choosing the conversation option or casting the spell, an Insight check must be passed, followed by a DC 18 Persuasion check. If succeeding both checks, the party can speak with Lann Tarv (with the same character that convinced Z'rell) to unlock the secret stash.
If Z'rell is still alive after killing or freeing the Nightsong, she leads the forces of the Absolute against the party during the assault on Moonrise Towers. She will start the combat 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.
It can be tempting to use a large Area of Effect spell to start combat like Fireball or Hunger of Hadar since Z'rell usually starts the fight in the center of a large, open entryway surrounded by her allies. However, this is a bait since she will use Counterspell on the first spell the party casts. Hence, it can be smart to cast a cantrip first to have her waste it and then use stronger spells.
is recommended. After choosing the conversation option or casting the spell, an 
check must be passed, followed by a DC 18 
check. If succeeding both checks, the party can speak with Lann Tarv (with the same character that convinced Z'rell) to unlock the secret stash.
and 
.
or 
since Z'rell usually starts the fight in the center of a large, open entryway surrounded by her allies. However, this is a bait since she will use 
on the first spell the party casts. Hence, it can be smart to cast a cantrip first to have her waste it and then use stronger spells.
Z'rell's model.
Z'rell on the Forgotten Realms Wiki