I hope I can do it justice. I may have to pre-write some of the speech stuff. A lot of the end (after the battle) is going to be "Ok, how do you handle X, Y, and Z over the next year or two?" I don't think they're going to keep exploring and find the lich who needs some help finishing his "Cleanse City" spell and go the genocide route, so there are going to be a lot of leftover Aarakocra to deal with somehow.

The lich been working on it for a couple of centuries, and will happily trade the party a Talisman of Pure Good for their help. It stopped working for him a while back for some reason, so he doesn't need it any more.

Spoiler: Cleanse City
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This epic-level spell takes 1 hour of Concentration to cast, and targets a point within 10 miles. Casting it involves a major visual display, enough to be visible from a distance and draw attention. The spell destroys all avian and mammalian life within a 5 mile diameter sphere. Affected creatures take 400 necrotic damage, CON DC 25 half. Lizard-like creatures, including yuan-ti, are not harmed by the spell. It is custom-designed to be cast by an undead yuan-ti. After casting, the caster suffers 3 levels of Exhaustion, even if normally immune to the condition, and is unable to cast any spells until the exhaustion has gone away.

The Nameless One will require substantial protection during and after casting, and magic of this level would definitely generate direct warnings to priests, as well as showing up in divinations.