A "curse seal" for good guys
Go to the gypsy & ask her to send you to the kyuubi dungeon (you know, that sewer-like place that Naruto's mind always wonder off to when he is in danger) She will do it for you only if you're good.
The dungeon is made out of many paths, traps & dead ends. Everytime you choose a path, you'll lose 1 turn. Once you've lost all your turns, HP (due to traps) or hit a dead end, you will get send back to the real world with full HP but loses all your turns.
The paths in the dungeon are fixed. Some lead you to the end faster & some takes you for a wild ride before spitting you out at the end of the dungoen. Along the dungeon you may get extra HP & turns if you're lucky.
Anyway, once you reached the end of the dungeon, you will have to fight Kyuubi. Beat him & you will obtain Kyuubi Seal.
Kyuubi Seal powers you up more than curse seal level 1 but less than curse seal level 2. It however, doesn't have any side effects.