Desire Demon

Whenever a creature enters the battlefield under
your control, you may gain 1 life.
Whenever a creature enters the battlefield under
an opponent's control, you may pay 1 life and tap Desire Demon. If you do, that creature can't
attack or block and it's activated abilities can't be activated until Desire Demon leaves the
battlefield. Desire Demon doesn't untap during
its controller's next untap step.