From everything we know in One Piece, Kaido is apparently unkillable. He’s tried to kill himself, he’s been “executed” but it always fails.
There are theories out there that Kaido is immortal because of the Ope Ope no Mi which would be an explanation for why he can’t die. The Ope Ope no Mi’s “ultimate technique” is to bestow immortality on a person at the cost of the user’s life.
Kaido has been shown to be practically invincible in the series only ever being injured once by Oden.
A lot of parallels are between Oden and Zoro but I doubt anyone really believes Zoro will be the one to defeat Kaido. Anyway my thought is that Law will not use his Devil Fruit power to grant immortality to someone but to take away Kaido’s immortality.
I don’t think Oda would bring up the ultimate technique of a Devil Fruit and never use it but I also think Oda likes to put twists into his story because well he’s a great mangaka. So my thought is that Law will sacrifice himself after Kaido reveals that he is truly immortal or they somehow find out that he is immortal due to the Devil Fruit power that Law has.
After Law sacrifices himself Kaido will still be a freaking monster because of his Devil Fruit and his ridiculous endurance/power/skills but he will be a mortal character now. The combined might of the alliance will be able to take Kaido down but Law will be the collateral.
*Theory by bkristensen92