2022-09-09 : If Monstrous War-Leech was kicked, it enters the battlefield with power and toughness each equal to the highest mana value among cards in your graveyard, including cards that were milled due to its replacement effect. For example, if you have no cards in your graveyard as it enters and you mill three land cards and a card with mana value 4, Monstrous War-Leech is a 4/4 creature entering the battlefield.

2022-09-09 : If a card in a graveyard has an {X} in its mana cost, X is 0.

2022-09-09 : Monstrous War-Leech’s second ability is a replacement effect, not a triggered ability. It happens at the same time that Monstrous War-Leech enters the battlefield, it doesn’t use the stack, and it can’t be responded to.