A friend of mine and I are playing mtg when he tried to counter my Crystaline Sliver with Final Reward. Is this a legal move?
I assume your Sliver is already on the board.
Final Reward needs to target a creature on the board.
Crystaline Sliver's shroud ability makes it that your Slivers can't be targeted.
So because you can't target Slivers, you can't remove it with Final Reward.
If you want to remove the Sliver, you would need something that doesn't target (like a Wrath of God) or something that would target the player and force him to sacrifice something (like Diabolic Edict)