Niblis of Frost — ruling 1

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.

Niblis of Frost : règles, fonctionnement et interactions (MTG)

Niblis of Frost is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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.

How it works : Niblis of Frost

Original English text — Wizards of the Coast / Scryfall

#1July 13, 2016· Wizards of the Coast
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.
No other card is named in this ruling.