Its funny that you mentioned that they don't seem immortal at all because the original effect of immortal was different but I thought it would be too powerful. I'll just post it anyway and you all can be the judge of that.
Immortal(beta version)
"Cannot faint from attacking moves"
-So as it says the Pokemon cannot faint from attacking moves but it can still take damage from them so the only way to faint them is through Entry hazards, statuses such as burn and poisoning, from Recoil damage they take or a special move like Destiny Bond or Perish song. I think that is all the ways to faint Pokemon without attacking, unless I am missing something. As you can see it's pretty broken kind why I avoided posting it.
New Move: Metamorphosis
Type:

/ Power:
-- / Accuracy:
--% / PP:
20 / Target:
All / Category:

/ Priority:
0
"Changes all moves to Status, Special or Physical"
-Yeah, Yeah I know the name is too long but this name is just to perfect for it. So the one it changes to is at random, so say
Metamorphosis is used it can turn all moves of the pokemon in battle into a a status, Physical or Special move This can really help with Tuanters and can turn the tide of battle, because a pokemon like Haxorus which has dreadful special attack would not benefit from all their moves turning into Special-based moves. Also if an attacking move turns into a status move they can no longer factor in Sp. Attack, Attack, Defense and Special Defense in attacking moves.
Distribution: Bug Types and Grass Types
New Move: Possess
Type:

/ Power:
-- / Accuracy:
--% / PP:
5 / Target:
Single adjacent Pokemon / Category:

/ Priority:
+5
"When this move is used, if the user faints, the Pokémon that landed the knockout hit will be unable to move in the following turn and the target's type becomes Ghost."
-Sorta like a last resort to guarantee safe switch in, so either they stay in or they have no choice but to switch. The Targets type will however remain Ghost until they are switched out.
Distribution: Ghost types.
Haunt
"Inflicts a curse upon fainting"
-Basically it inflicts the Curse status(or whatever it is) on the Pokemon that fainted the Pokemon with this ability
Distribution: Ghost Types