magic the gathering: waterknot?

You'd want to play it on an opponent's creature, yeah. It locks a creature down and makes it mostly useless.

So you cast it on a target creature, and on resolution, the aura enchants it.

When Waterknot resolves to the field, the creature it enchants becomes tapped. Then when the controller's turn reaches the untap step, that creature doesn't untap. So it can't be used to attack with, or block, and if it had an ability that requires it to tap, then it would be unable to use it - unless he can untap it with a spell or ability, or remove Waterknot.