In Act 2 of Baldur's Gate 3, Wyll's past quickly catches up to him in the form of Mizora, the succubus who serves as his fiendish warlock patron, when she makes an unexpected appearance at your camp to strongarm you into her personal quest.
If you want to save Wyll from the harrowing depths of the Nine Hells, Mizora's task for you is clear: retrieve Zariel's asset from the Moonrise Towers. Unfortunately, however, this asset in question remains inaccessible until you complete a significant part of Act 2.
Where To Find Zariel's Asset
Both the "Rescue Zevlor" and "Find Zariel's Asset" quests can be completed in roughly the same part of the Mindflayer Colony under the Moonrise Towers.
What the quest log doesn't make clear is that the Mindflayer Colony cannot be accessed until after Ketheric Thorm is defeated for the first time on the rooftops of Moonrise Towers, so don't waste your time before then!
Once Ketheric has made his escape deeper into the towers, however, follow him and make your way to one of the northern-most chambers (X:674, Y:42)
You'll want to travel past the Tadpoling Center where you can rescue Zevlor, to find a Mindflayer Pod containing none other than Mizora. Turns out, she was Zariel's Asset all along—that explains why she was so eager to see it saved.
Check the nearby brine pool for a Mindflayer Parasite Specimen!
How To Rescue Zariel's Asset
Once you've found Mizora, your options are as follows:
Release Mizora
Choosing either the "[Strength] Gather your strength and smash your foot through the pod" option, or reading the Mindflayer runes to decipher the "Unleash" option, will release Mizora from her pod.
She will begrudgingly agree to release Wyll from his contract, but only after six months, in accordance with the terms and conditions of the contract.
Nothing you say will change her mind here, but you can push further to request a bigger reward from her. A DC 14 Persuasion Check will see her giving you an Infernal Rapier, which comes with some interesting perks.
When wielding it, you'll be able to add your spellcasting modifier to attack rolls, opening up the possibility for spellsword and gish builds. It'll also give you access to the sixth-level conjuration spell, Planar Ally: Cambion—this is a very, very powerful summoning spell.
Kill Mizora
Choosing the "Annihilate" option instead will kill Mizora in her pod.
We've learned from Yurgir previously, however, that devils cannot die on the Material Plane, and will instead be sent back to the Nine Hells. This breach of contract will drag Wyll into the Hells, without truly killing Mizora. Wyll will be disintegrated, and your companions will lament the pile of ashes that is left of him.
We don't recommend choosing this option unless you really, really hate Wyll.