It seems like the hollow-like creature rampaging in the human world is indeed one of Aizen’s experiments. Some people have speculated that it could be one of the Vasto Lorde, but as we know close to nothing about them, aside from the fact that Harribel is one, it’s difficult to confirm anything. The monster is able to fight toe to toe against Isshin, and as he noticed, its style is much like a shinigami’s (or as we know it, arrancar/vizard). The first of the two main developments in this chapter is Aizen and his crew showing up and attacking Isshin right after he releases his shikai against the creature. Isshin’s zanpakutou Engetsu bears some similarities to Zangetsu, from the moon theme to Getsuga Tenshou, but it also seems to have some fire-based properties. If I recall correctly, this would make him the only shinigami shown to have a fire-based zanpakutou other than Yamamoto.
The other big thing that happend would be Masaki rushing off to help after she feels the two reiatsu’s battling, despite Ryuuken’s stern warning about risking themselves meddling in shinigami affairs. Of course, she doesn’t give a crap about it and decides to go anyways, and unsurprisingly, Ryuuken decides to follow. I’m wondering whether the scene at the very beginning of the flashback was actually the same as what happens here, when Masaki first meets and rescues the injured Isshin. Everything lines up, and it would also mean that Masaki was able to defeat the creature and possibly cause Aizen to back off. It’ll be interesting to see what types of Quincy abilities she will be using.