Ruvas, the Malignant

Haste, First Strike, Menace
Ruvas, the Malignant must be blocked if able
Ruvas, the Malignant enters the battlefield with a number of +1/+1 counters on it equal to the amount of mana spent to cast it.
When Ruvas, the Malignant attacks each hydra you control gains +X/+X until end of turn, where X is equal to the number of hydras you control