You were an Anbu from Konoha and got caught in Orochimaru's lair. He took off your mask and said that if Sasuke's body doesn't suit him, you will be his second choice.

It leaves a cursed seal technique bite on you.

He licks his lips with his long tongue. "That's right.. This seal will give you extra chakra and a couple more details... Huh, and after that, only one in ten survive" he sighs and strokes your face.