You'll want to tackle wandering in the dark right after finishing up with keira in hunting a witch, it's just the natural flow of things. The quest itself is pretty intense, and i learned the hard way to stock up on cat potions before diving in those dark elven ruins are no joke. Working with keira is actually pretty interesting, and you'll get some really important story bits about ciri and the wild hunt while you're down there.

Something worth mentioning is to bring plenty of swallow potions and maybe even some food, cause the fights can get pretty rough. You'll run into some nasty drowners, and there's this golem that gave me trouble until i figured out elementa oil works great on it. The whole quest really opens up some cool stuff about portals and magic, which comes in handy later on.

Good to know that while you can technically do some other stuff first, this quest really sets up important story pieces that make later parts make more sense. Plus, the rewards are definitely worth pushing through those dark caves just remember to keep your gear repaired cause it's quite the journey down there.