Whirlpool Whelm should have a 'may' ability.

[FranAvalon]

When I win the clash, I should choose whether to put the creature in the opponent's hand or on top of the library. Instead, it automatically puts the creature on top of the library if I win.

"Clash with an opponent, then return target creature to its owner's hand. If you win, you may put that creature on top of its owner's library instead."