Niblis of Frost

Flying
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever you cast an instant or sorcery spell, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.

Pioneer Masters

Commander 2020

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If multiple effects say that a creature doesn’t untap during its controller’s next untap step, those effects all apply during one untap step. For example, if you cast two instants in one turn and target the same creature twice with Niblis of Frost’s ability, that creature will only spend one untap step without untapping.