You may choose not to untap Sarlacc during your untap step.
Choose one of your opponent's Characters. As long as Sarlacc remains tapped, the chosen Character doesn't untap during the untap step, and at the start of the Character battle step, put 2 damage counters on the chosen Character. Play only when Sarlacc would attack.
Text erratum
Once the Sarlaac becomes tapped, the targeted unit takes 2 damage at the start of each Character battle as long as Sarlacc remains tapped regardless of whether the unit it taps becomes untapped.
As soon as the Sarlacc untaps (whether from your normal untap step or from some other card), its effect ends.