Phyrexian Holder

Infect
{b}{b}, {t}: Tap target creature. It cannot untap as
long as Phyrexian Holder is tapped. During your
untap phase, you may choose to keep Phyrexian
Holder tapped. If you do, Phyrexian Holder deals
1 damage to the tapped creature and to itself.
It's just you and me now. Neither can live while the
other survives, and one of us is about to leave for good."