Aven Heartstabber

If you have five or more mana values among cards in your graveyard and then some of them are removed such that you have fewer than five mana values among those cards, Aven Heartstabber loses its bonus to power and toughness. Notably, this may cause it to die if it now has damage marked on it greater than its toughness.

“Five or more mana values among cards in your graveyard” means there are at least five different mana values among those cards.