Whether this card sees play would depend on its synergy within the deck's overall strategy and power level compared to other available options
Nightmare was a core set staple since Alpha , and its probably better in mono-color decks and in environments that let you run multiples
If you needed to deal 40 damage to everyone else, now it's 20
This is, obviously, a rather narrow ability
Drafted packs and reward packs are of course not the same - drafted packs have 1 R/M, 3U, 10C, while reward packs have 1 R/M with a 1/12 chance of a wildcard, 2 U, 5 C
You do need to have some amount of board presence established for this to end the game, but it doesnt take too many creatures for this to overwhelm a table beneath a flurry of hooves, paws, and other things to make a lake of your table