TMdict

Howl of the Wounded Beast. The continuously active Noble Phantasm of Berserker of “Red,” Spartacus. It is the type where his legend itself was sublimated and turned into a Noble Phantasm. There is actually a huge difference in how this Noble Phantasm is used depending on whether he is summoned as a Berserker or a Saber. When he is summoned as a Berserker, it turns the wounds he receives from enemies into mana and accumulates it in his body to boost or heal himself.
However, during the Great Holy Grail War, perhaps due to his connection to the present world—his line being complex, the Noble Phantasm’s conversion rate went out of control and turned him into something unbelievable.
Incidentally, if he was summoned as a Saber, it would have stabilized into the more respectable ability of “restoring his vitality and mana when he successfully endures the enemy’s attacks and also invalidating or reflecting back any more of the same attacks thereafter.” Though this would have been threatening enough on its own.
Either way, the longer a battle is prolonged, the more advantageous it is for Spartacus. The only way to beat him is in a short decisive battle.