Quote Originally Posted by paulbee View Post
Wasn't it stated somewhere that only Madara could summon the Kyuubi, if that is true would it be some kind of contract (willing or unwilling), or whether the Kyuubi's chakra just somehow interferes with the summoning attempt.
Technically Obito can summon it as well, along with Gedou Mazou as well (but it seems that only after he obtained Rinnegan eye).

I believe that there is a contract formed the same way it is with other summoning animals. The same ritual is performed in order to summon it after all. Of course the contract is done unwillingly, but with the MS's powers, it is quite easy to form it on the spot. Rather, MS's properties seem to allow to form a summoning contract without need of the blood contract. Even Sasuke formed a contract with a hawk, by seemingly only using his MS (hax ). Perhaps it is then done on spiritual level, by sharing a bit of their spiritual energies, rather than through blood, which seems to have rather physical aspects. This might suggest that it was the Elder Brother's or his descendants that first formed summoning contracts with animals. This isn't surprising at all, because it's another watered down power of the Rinnegan's Animal Path, that Sharingan/MS inherited.