With a slight modification to Urahara’s tech, Aizen, Tousen and Gin are able to sport invisibility coats and take cheap shots at Isshin. Given what we know about Aizen’s shikai abilities, they seem excessive and a bit overpowered at the same time. Isshin is unable to focus on figuring out what hit him besides the fact that it was from a shinigami (though I’m sure he would be able to counter given the chance) so they decide to let his fight against the creature play out after crippling him. As it turns out, the monster is indeed some sort of hollow-shinigami hybrid and is probably more advanced than the vizards. It’s a very fast, beserker-type fighter that completely disregards its wounds and seems incapable of speech. The similarities to Ichigo’s hollow are undeniable, especially to the powerful form that manifested itself against Ulquiorra back in the arrancar arc.
In the end, Masaki shows up and begins firing arrows at the beast, to no avail. She quickly changes tactics however, and manages to blast its face off after luring it into a close range attack. It’s a pretty clever and impressive move, and I feel like she managed to end the fight in no more than ten seconds. No wonder Isshin was embarrassed. I doubt that Aizen and his crew will reveal themselves here, especially since Isshin already knows someone is there, and they might just leave after Ryuuken shows up as well. Aizen seems to be curious about the Quincies as well, so he probably won’t interfere just yet. Perhaps this was the hollow attack that the Fullbringers were referring to.