Those who go under rise again as cruel mockeries of their former selves, spewing foul water and keening with sorrow.

Terror Tide

Fathomless descent — All creatures get -X/-X until end of turn, where X is the number of permanent cards in your graveyard.

The Lost Caverns of Ixalan Promos

The Lost Caverns of Ixalan

Cards with the ability word "fathomless descent" have abilities that care how many permanent cards are in your graveyard.

Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.

The value of X is checked only once, as Terror Tide resolves.