This spell costs {1} less to cast for each creature that attacked this turn.
Witchstalker Frenzy deals 5 damage to target creature.
Witchstalker Frenzy's first ability can't reduce its cost to less than {R}.
Witchstalker Frenzy's first ability doesn't change its mana cost or mana value, only the total cost you pay. Specifically, the mana value of Witchstalker Frenzy is always 4.