Mortarion, Daemon Primarch — ruling 1

Mortarion, Demon Primarch's ability looks only at the amount of life you've lost this turn, regardless of any life you may have gained. For example, if you lost 3 life due to combat damage, but gained 2 life later in the turn, X may be up to 3 for this ability.

Mortarion, Daemon Primarch — ruling 2

If a spell or ability sets a player's life total to a specified amount, that player gains or loses the appropriate amount of life. For example, if your life total is 20 and an effect sets your life total to 15, you lost 5 life.

Mortarion, Daemon Primarch : règles, fonctionnement et interactions (MTG)

Mortarion, Daemon Primarch 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 Primarch of the Death Guard — At the beginning of your end step, you may pay . If you do, create X 2/2 black Astartes Warrior creature tokens with menace. X can't be greater than the amount of life you lost this turn.

How it works : Mortarion, Daemon Primarch

Original English text — Wizards of the Coast / Scryfall

#1October 7, 2022· Wizards of the Coast
Mortarion, Demon Primarch's ability looks only at the amount of life you've lost this turn, regardless of any life you may have gained. For example, if you lost 3 life due to combat damage, but gained 2 life later in the turn, X may be up to 3 for this ability.
No other card is named in this ruling.
#2October 7, 2022· Wizards of the Coast
If a spell or ability sets a player's life total to a specified amount, that player gains or loses the appropriate amount of life. For example, if your life total is 20 and an effect sets your life total to 15, you lost 5 life.
No other card is named in this ruling.