About halfway through the War, the Holy Grail was usurped and corrupted by a third party. At that point, my contract with Berserker was severed, and I fell to the Far Side of the Moon, where Gilgamesh was sleeping. The Moon Cell had ruled that he was too overpowered to be put into circulation as a normal Servant, but... well, I got lucky.

Before my 'right to exist' was revoked because I lost Command Seals, I managed to interest him enough for him to extend his hand to me. And he stuck with me after we resolved the Grail issue, and we went on to win together.