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, 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.