Receiver & Power of Alchemy are Pokémon abilities that give the Pokémon the ability of their fallen teammate. The former is the signature ability of Passimian while the later is signature ability of the Alolan Muk family.
When an allied Pokémon faints in a double or triple battle, the Pokémon with this ability gains their ability. It will activate as if the user was just sent out. It will still activate regardless of if the original user was fainted by its own move or by the one with this ability.