Festive Funeral

Instant

Target creature gets -X/-X until end of turn, where X is the number of cards in your graveyard.

"Even as we mourn the last, we look to the next with joy. Hopefully, my new suitor will bring the Cauldron home."
—Ayara, First of Locthwain

2019-10-04 Festive Funeral is still on the stack while you determine the value of X. It doesn't contribute to the value of X.
2019-10-04 The value of X is determined only as Festive Funeral begins to resolve. The target creature's power and toughness won't change if other cards are put into your graveyard later in the turn.

Card is in preconstructed decks:

mtg.wtf