Board Thread:Mortal Kombat X/@comment-25033529-20150505005712/@comment-12973094-20160207145910

If I remember right, Raiden relinquished his immoratality to aid Earthrealm during the first tournament. Than, when the Elder Gods were attacked by Shinnok during MK4, Raiden became a Elder God, replacing a deceased deity. Again, during the Deadly Alliance, Raiden gave up his immortality again to aid the heroes against the might of Quan Chi and Shang Tsung.

He may still have a kind of immortality, like he doesn't die of old age or diseases, but he can be killed, just like an elf or vampire (please, I didn't said that he's an undead).

Shinnok, on the other hand, have true immortality, as we witnessed when his head was still alive separated from his body.