Merieke Ri Berit doesn't untap during your untap step.
: Gain control of target creature for as long as you control Merieke Ri Berit. When Merieke Ri Berit leaves the battlefield or becomes untapped, destroy that creature. It can't be regenerated.
(25/09/2006)
If another player takes control of Merieke, you will immediately lose control of the creature you took, but Merieke’s “destroy” ability won’t trigger. The affected creature will go back under the control of the appropriate player.
(25/09/2006)
If Merieke becomes untapped, its “destroy” ability will trigger, but you won’t lose control of the creature you took before it’s destroyed.
(25/09/2006)
If Merieke leaves the battlefield, you will immediately lose control of the creature you took and Merieke’s “destroy” ability will trigger. The affected creature will go back under the control of the appropriate player, then it will be destroyed.
