So, as we know, Demons can’t truly die unless killed in the Twisting Nether, but the ways we’ve seen demons brought back vary. So, I wanted to list the different means of demon resurrection here as a resource for Demon Hunter, and Warlock players (or, really anyone who roleplays a character who uses fel.)
1: The OG. Any demon (and some Demon Hunters) killed outside of the influence of the Twisting Nether can simply regenerate in the Twisting Nether over time. How long it takes isn’t certain, but the lore does say that the resurrection can be sped up (more info in #2 and #3). It could be anywhere from weeks to decades.
It also probably depends on the demon. (In-game warlocks can resummon their demons instantly after death, but in-game function isn’t the best source for lore.) Highlord Kruul took at least a few years to regenerate, but his regeneration was also incomplete. Still, he was a far lesser demon than something like an imp, felhound, or succubus would be, so it probably doesn’t take anywhere close to as long for them.
For Demon Hunters powerful enough to survive death outside of the Twisting Nether, I would assume it would be somewhere between that of a powerful demon, and a lesser demon, but there is no concrete lore on the time it takes so you can really do whatever you want. (Though people might not want to roleplay with you if you are dying and resurrecting every day. Make it take at least a few IRL months FFS.)
2: Getting Loramus’d. Loramus Thalipedes was a Demon Hunter who was killed and cut into four separate pieces. Once the pieces were brought back together he was resurrected using “demonic energies” from the Twisting Nether, through the use of a Demonic altar. This implies that despite the injuries, as long as you have access to an altar like it, and the full body of he demon, you can heal them and resurrect them this way. That said, this can be taken with a grain of salt since all of the lore on it comes from a single quest in game, and this method isn’t seen anywhere else.
3: Getting Varimathras’d. This can’t happen anymore, since Argus is dead and Sargeras is imprisoned, but there are two things that could have been happening because of Argus, and/or Sargeras’ presence in the Twisting Nether. What we do know is that Antorus is used as an anchor point to pull demons from their peaceful regeneration in the Twisting Nether, to Antorus, where their regeneration was accelerated. This is confirmed both in-game, and in A Thousand Years of War.
Another possibility is that the presence of Argus allowed demons to return to life EVEN IF they died in the Twisting Nether. This is just speculation based on a particular, infamous quote from Turalyon.
It is also worth noting that Demon Hunters could be found and brought to Antorus too. The demon hunters in question either joined the Legion, or died.
Hope you found this useful guys.