That would be "To Baator and Back". Synopsis: PCs hear about some people that went missing after searching through "the old wizard's" home. While exploring, PCs trigger a portal that dumps them in Avernus. A few short encounters to get that planar feeling and the adventure ends with the PCs making it to Sigil.
Oh right...I never liked the ending to that adventure. 'You have to bring this with you to escape, but it just turns to dust on the other side even though it's obviously sinister.'
