Because it's a flame type Zanpaktou...Damn that Aizen. The fight was just about to get to the good part. Isshin's shikai release sounded interesting, don't know why it was 'burn' though.
No one said it was his Bankai.That wasn't his bankai, that was his shikai.
I doubt its a experiment....its a Vasto lorde remember Aizen was hunting for one? And I bet that we see ALOT more of engetsu, remember Isshin knows of the Final Getsuga Tenshou so that tells me that after Masaki gets involved she helps him battle the vasto lorde and gets hurt by him while she is pregnant with Ichigo hense Ichigo getting his hollow powers like the Fullbringers did. My guess is this wraps up with A long drawn out battle over a few years and its finshs with Urahara helping isshin distract Aizen so that Isshin may use to the Final Getsuga Tenshou to save Masaki and kill the Vasto Lorde, hense Aizens obsession with Ichigo.I said a while back that Engetsu was a Fire based Zanpakuto and i got dismissed quite quickly lol. Just saying i knew i read it right in the manga a while back that it was
Also i think this chapter was a bit of a mind blower... a "Black Hollow"... quite unexpected detail there and the fact most everyone's major theory that Masaki was the Hollow is now down the drain.
Well i guess its safe to assume the Hollow was a Shinigami Hollow Experiment...so im going to go ahead and think that this hollow is Kaien. ( i know everyone will throw out that idea, as its more or less hopes and my thought that Kaien had a lot larger role than just being someone Rukia cared for and attack by a hollow that consumed him).