The Balrog, Flame of Udûn — ruling 1

The Balrog, Flame of Udûn will be put on the bottom of its owner's library only if it's still on the battlefield when the ability resolves. If it leaves the battlefield before that point, it will stay in whatever zone it's in, even if it's returned to the battlefield before the ability resolves.

The Balrog, Flame of Udûn : règles, fonctionnement et interactions (MTG)

The Balrog, Flame of Udûn 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

Trample When a legendary creature an opponent controls dies, put The Balrog on the bottom of its owner's library.

How it works : The Balrog, Flame of Udûn

Original English text — Wizards of the Coast / Scryfall

#1June 16, 2023· Wizards of the Coast
The Balrog, Flame of Udûn will be put on the bottom of its owner's library only if it's still on the battlefield when the ability resolves. If it leaves the battlefield before that point, it will stay in whatever zone it's in, even if it's returned to the battlefield before the ability resolves.
No other card is named in this ruling.