Because Ghastly Demise isn't put into your graveyard until it's finished resolving, it won't be counted when determining whether the creature is destroyed.
10-11-2020
wotc
Ghastly Demise can target any nonblack creature. Compare the creature's toughness to the number of cards in your graveyard as Ghastly Demise resolves to see if that creature is destroyed.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.