Larian Studios

Devils, or really any planar creature, slain outside of their home plane are returned to their plane of origin. In the case of summoning, they return intact, unharmed and likely rather annoyed at the mortal who wasted their time - Which could have been a few minutes of combat or hundreds of years bound guarding a room.
If the planar being in question was NOT summoned and had freely travelled to non-native location, be it by portals, plane-shift spells or other similar means, they're in for a bit of a rougher time.
Some planar entities have to reconstitute themselves, which can be a timely and painful process unless they've got contingencies in place. Others can be reduced to the lowest form of their particular planar subtype, like Lemures and Nupperibos and need to re-ascend their particular hierarchies.
Now, if they're slain on their home plane - That's it. Death. Oblivion. The End. Roll Credits.

Rafael, however, is *not* a devil.
Well, not a full devil.
While he is the son of Mephistopheles, he's considered a Cambion, which is a half-fiend. That little tidbit lets him skirt the rules when it comes to his death should you elect to kill him in the House of Hope. Strictly because he's a cambion and NOT a devil, his soul goes to Baator/Hell, which is where Daddy-Dearest comes into play.
Mephistopheles makes a point in acquiring Rafael's soul and subjecting him diabolical torment for his many betrayals, which you can get a glimpse of in the scrying orb at the Diabolist's shop after returning from the House of Hope.

So regardless of where he is killed - Hell or Faerun, his soul or essence is going back to hell and, outside of special interest from greater powers - Mephistopheles, in this case - he'd likely be a Lemure or Larvae at best.