A devourer creates either a zombie, ghoul, or wight, all undead; and a spawn of Kyuss created another spawn of Kyuss, also undead.
You should kill or destroy the undead created by the devourer or spawn of Kyuss, and then cast true resurrection on the body or by speaking their name, whichever is appropriate.
True resurrection says:
You touch a creature that has been dead for no longer than 200 years and that died for any reason except old age. If the creature’s soul is free and willing, the creature is restored to life with all its hit points.
This spell closes all wounds, neutralizes any poison, cures all diseases, and lifts any curses affecting the creature when it died. The spell replaces damaged or missing organs and limbs. If the creature was undead, it is restored to its non-undead form.
The spell can even provide a new body if the original no longer exists, in which case you must speak the creature’s name. The creature then appears in an unoccupied space you choose within 10 feet of you.
True resurrection is quite clear, “If the creature was undead, it is restored to its non-undead form“, since the devourer and the spawn of Kyuss both create undead from your fallen comrade, true resurrection will work to bring them back to their previous form.
Now, what if for some reason your 17th-level or higher party of adventurers cannot come up with 25,000 gp for the diamonds required for true resurrection? We are back to needing a wish spell – we still need not depend on the whims of our DM. Wish can create the diamond we need:
You create one object of up to 25,000 gp in value that isn’t a magic item. The object can be no more than 300 feet in any dimension, and it appears in an unoccupied space you can see on the ground.
Then we can use that diamond to cast true resurrection. No need to worry about a scheming DM throwing some monkey’s paw at our wish.