Fiends and devils killed on the material plane typically will just reform in the Nine Hells or the Abyss. MToF has a lot of info on fiends, devils, and demons. There are some places, like Avernus, where dying means destroyed, but not so on the material plane.
It also depends on the setting. In Eberron, Rakshasa and many other fiends are native to the material plane and cannot be slain permanently; they reform after a period of time. This makes the Lords of Dust very interesting as opponents.
But as others indicated in MTOF, fiends killed on their home plane (Devils in Baator, Demons the Abyss, Yugoloths in the Gray Waste) normally are dead per settings that use the great wheel. Same would apply to Angels on Mount Celestia
In prior editions, there were comments that if a Demon Lord/Arch Devil were slain on the prime material, they would be unable to be resummoned to that plane, but that's not mentioned in the current edition AFAIK.
As far as Cambions, being a half-fiend it isn't clear as written. So it depends; if you think they are mortal, then death is death anywhere. If they are immortal, then pick a place they can be killed.
If they are killed in the material plane do they reform in a lower plane or are they just dead?
Fiends and devils killed on the material plane typically will just reform in the Nine Hells or the Abyss. MToF has a lot of info on fiends, devils, and demons. There are some places, like Avernus, where dying means destroyed, but not so on the material plane.
It would depend on the nature of the fiend parent.
It also depends on the setting. In Eberron, Rakshasa and many other fiends are native to the material plane and cannot be slain permanently; they reform after a period of time. This makes the Lords of Dust very interesting as opponents.
But as others indicated in MTOF, fiends killed on their home plane (Devils in Baator, Demons the Abyss, Yugoloths in the Gray Waste) normally are dead per settings that use the great wheel. Same would apply to Angels on Mount Celestia
In prior editions, there were comments that if a Demon Lord/Arch Devil were slain on the prime material, they would be unable to be resummoned to that plane, but that's not mentioned in the current edition AFAIK.
As far as Cambions, being a half-fiend it isn't clear as written. So it depends; if you think they are mortal, then death is death anywhere. If they are immortal, then pick a place they can be killed.