Nihiloor — ruling 1

As Nihiloor's first triggered ability resolves, you choose up to one creature for each opponent, and tap them all at the same time. Then, a separate triggered ability is put on the stack for each creature you tapped this way. Those abilities are placed on the stack in the order you choose. If tapping those creature caused any other abilities you control to trigger, those are also placed on the stack at this time.

Nihiloor — ruling 2

Both the tapped creature's power and the target creature's power are checked both as you put the ability on the stack and as the ability resolves in order to make sure the target is legal.

Nihiloor — ruling 3

If the tapped creature is no longer on the battlefield as the trigger resolves, use its last existence on the battlefield to determine whether the target is still legal as the ability resolves.

Nihiloor — ruling 4

If a creature you tapped for Nihiloor's ability is somehow no longer tapped as the reflexive triggered ability resolves, that ability will still resolve and you will still gain control of the target as long as it has less power than the creature you tapped.

Nihiloor — ruling 5

If Nihiloor leaves the battlefield before its enters-the-battlefield ability resolves or before its reflexive triggered abilities resolve, then you won't gain control of any creatures.

Nihiloor : règles, fonctionnement et interactions (MTG)

Nihiloor 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

When Nihiloor enters, for each opponent, tap up to one untapped creature you control. When you do, gain control of target creature that player controls with power less than or equal to the tapped creature's power for as long as you control Nihiloor. Whenever you attack with a creature an opponent owns, you gain 2 life and that player loses 2 life.

How it works : Nihiloor

Original English text — Wizards of the Coast / Scryfall

#1July 23, 2021· Wizards of the Coast
As Nihiloor's first triggered ability resolves, you choose up to one creature for each opponent, and tap them all at the same time. Then, a separate triggered ability is put on the stack for each creature you tapped this way. Those abilities are placed on the stack in the order you choose. If tapping those creature caused any other abilities you control to trigger, those are also placed on the stack at this time.
No other card is named in this ruling.
#2July 23, 2021· Wizards of the Coast
Both the tapped creature's power and the target creature's power are checked both as you put the ability on the stack and as the ability resolves in order to make sure the target is legal.
No other card is named in this ruling.
#3July 23, 2021· Wizards of the Coast
If the tapped creature is no longer on the battlefield as the trigger resolves, use its last existence on the battlefield to determine whether the target is still legal as the ability resolves.
No other card is named in this ruling.
#4July 23, 2021· Wizards of the Coast
If a creature you tapped for Nihiloor's ability is somehow no longer tapped as the reflexive triggered ability resolves, that ability will still resolve and you will still gain control of the target as long as it has less power than the creature you tapped.
No other card is named in this ruling.
#5July 23, 2021· Wizards of the Coast
If Nihiloor leaves the battlefield before its enters-the-battlefield ability resolves or before its reflexive triggered abilities resolve, then you won't gain control of any creatures.
No other card is named in this ruling.