1st when I met her.Karlach is awesome. The nicest rage-aholic you could ever hope to meet. I love the little dance she does when the party is hanging around not doing much. Her back story is overwritten. Like, a lot. She's a former senior enforcer of an Arch Duke of Hell, who's on the run. Before that she was the chief enforcer of an underworld figure who sold her into slavery. She's a whath level barbarian when you meet her? Maybe 3rd?
For converting her as an NPC for one of my 3.5e Greyhawk campaign (just getting underway with a third spin-off from the ongoing original campaign), my explanation is she was Level Drained. That’s the AD&D term. In 3x terms, Negative Levels that she failed saves on so they became permanent. (Not sure that’s even a thing in 4e or 5e. Darned convenient for a DM melding characters from different levels from different campaigns into reasonable compatibility in Older Editions.)
My explanation for how she got from FR to Avernus to Greyhawk is Dismissal during the Blood Wars. By 3x rules (at least), Dismissal sends a creature to their native plane, and Karlach is from the Prime Material Plane (variations on the name by edition exist, because “reasons”). But the PMP has multiple worlds - Dismissal from Avernus to the PMP can (as I read it and run it) land you on the “wrong” world, and anywhere on it. The rules also call for a 30% of the entirely wrong plane, so wrong “crystal sphere” doesn’t feel like a stretch to me.
On the plus side for everyone’s favorite Tiefling, I’d say no tadpole nor Nautiloid, since my campaign will not be about that.
On the minus side, I’ve already established that Sword Coast Common and Greyhawk Common are different languages, but I would give her Infernal as a free language. So she has to use gestures and grunts to communicate until she levels up to 2nd and can use Skill Points to learn the local Common. Or needs an Infernal speaking translator. (Tongues isn’t going to exist in a low level group.). Which is actually a lot of fun, imho.