Thalakos Dreamsower [Tempest]

Shadow (This creature can block or be blocked by only creatures with shadow.)
You may choose not to untap Thalakos Dreamsower during your untap step.
Whenever Thalakos Dreamsower deals damage to an opponent, tap target creature. That creature doesn't untap during its controller's untap step for as long as Thalakos Dreamsower remains tapped.