And the activation of the Magic Circuits, in Matō Kariya's case, means a hell of pain by having his body eaten by the Crest Worms.

When dematerialized, the prana consumption is at its minimum. Still, in that state, Kariya would sometime be tortured with palpitations and dizziness.

But when Berserker was materialized, the agony was beyond imagination.

The foreign body awakens, starts eating his flesh, creaking his bones. Acting as Kariya's pseudo Magic Circuits, the Crest Worms supply the sucked off prana to Berserker without concern for the host's limits.

Speaking about agony isn't even half of it. Being corroded, plundered by a living creature inside his body —the pain of being devoured alive doubles with fear and disgust.

"Guh... Ga, gwa...hh!!"

Hiding in darkness, Kariya stifled his scream of impending death, raking up his throat and chest. When his skin tore and blood started running, he started chewing off his nails on both hands.

For even more of a disaster, the prana consumption required from the Master by the Berserker class is even higher. When Zōken had Kariya use the maddening formula during the summoning to make him stronger, maybe this really was already a plan of the old magus to tyrannize him.

The worms bite at his spine. The worms are melting his nerves. The worms are, nesting en masse inside Kariya's body, the worms are the worms are the worms are the worms the worms the worms the worms...

"Gaaaaahh..."

He was unable to hold out his scream, but only a feeble moan came out. The intense pain was choking him and wouldn't come out of his throat. As he started sobbing, Kariya kept trampling his body out of rage.

- Fate/Zero: Act 4, Part 3