What is this kizu ?

[Jon]

Just been looking at some cheaply auction blades and came across this really interesting kizu, but I’m not sure what it actually is  apart something that looks really nasty.

[Ray Singer]

Looks like it's simply an open side to a fukure (or just a kitae-ware). It's not that uncommon to see flaws around the area of the habaki-moto where the blade transitions to the steel of the nakago.