Fiends Respawning

Most Devils (Diabolical Restoration), Demons (Demonic Restoration), and Yugoloths (Fiendish Restoration) have the ability to respawn in their home if killed outside of their home plane. This has the rather odd effect that, if two demon lords decide to get into a war, they're much better off hiring a bunch of yugoloths (who can be trivially rehired if slain) than using demons (who will stay dead), and that in general fiends in their home plane would be likely quite reluctant to actually fight.

Back in the AD&D era it took quite a while for a killed demon to acquire a new body that it could use outside of its home plane so this didn't really make much difference, but 5th edition has done away with that, which makes me want to come up with some other way to make it so it actually makes sense for demons to fight in the abyss, etc. My tentative thought is to just change the restoration from

Demonic Restoration. If the demon dies outside the Abyss, its body dissolves into ichor, and it gains a new body instantly, reviving with all its Hit Points somewhere in the Abyss.

to something like

Demonic Restoration (recharge 6 after a long rest, or when summoned). If the demon dies, its body dissolves into ichor, and it gains a new body instantly, reviving with all its Hit Points in its lair (if it has one) or another appropriate location in the Abyss. While this power is on recharge, the demon is incapable of leaving the Abyss.

but I'm wondering if there's any lore about how restoration works.

I think the demon lords can afford to use demons because there's just a LOT of them. Like, yeah, thousands of demons are dying all the time in these huge, terrible demon wars, but there's so many anyway that it doesn't really matter, and they can always wait a few hundred years to resupply before they continue their wars.

Because it's quite difficult for them to leave their homeplane in the first place. The reason is somewhere in the lore but if I remember correctly, at least for devils, it was because asmodeus made some deal with the angels.

Fiends usually require some sort of summoning to be able to leave their plane afaik.

Somewhere in my noggin the phrase "a year and a day" is hanging around. I'm not sure where I heard it, whether it was some prior D&D material or just some random source somewhere else. But I think it's got a nice ring to it, and I think that's a fair limitation to the fiendish restoration thing. That being said, fiendish leaders really don't care about the health or safety of their minions, and are happy to throw a few thousand of them into the meat grinder of war on a whim, with no regard for their suffering. 

Well, devils are likely a bit more responsible about it- gotta be able to justify it to your own boss unless you want to get an up-close view of the meatgrinder. But yeah, as long as they expect to get something from it- possibly even just gratification- powerful fiends aren't going to be shy about expending minor fiends.