Ulamog, the Ceaseless Hunger — ruling 1

Ulamog's first ability resolves independently of Ulamog once they've both been put on the stack. If Ulamog is countered, that triggered ability will still resolve. That triggered ability will always resolve before Ulamog does.

Ulamog, the Ceaseless Hunger — ruling 2

Cards exiled by Ulamog's first and third abilities are exiled face up.

Ulamog, the Ceaseless Hunger — ruling 3

If the player has less than twenty cards in their library, exile all of them. That player won't lose the game until they have to draw a card from an empty library.

Ulamog, the Ceaseless Hunger : règles, fonctionnement et interactions (MTG)

Ulamog, the Ceaseless Hunger 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 you cast this spell, exile two target permanents. Indestructible Whenever Ulamog attacks, defending player exiles the top twenty cards of their library.

How it works : Ulamog, the Ceaseless Hunger

Original English text — Wizards of the Coast / Scryfall

#1August 25, 2015· Wizards of the Coast
Ulamog's first ability resolves independently of Ulamog once they've both been put on the stack. If Ulamog is countered, that triggered ability will still resolve. That triggered ability will always resolve before Ulamog does.
No other card is named in this ruling.
#2August 25, 2015· Wizards of the Coast
Cards exiled by Ulamog's first and third abilities are exiled face up.
No other card is named in this ruling.
#3August 25, 2015· Wizards of the Coast
If the player has less than twenty cards in their library, exile all of them. That player won't lose the game until they have to draw a card from an empty library.
No other card is named in this ruling.