It's an unreleased OST called *Soul Society* by *Bleach*'s music producer, Shiro Sagisu.

The relevant [YouTube link](

Quote from OP:

> I found it. Can't believe I found it. It was so hard that even in the comments people say that found it by mistake and was not on purpose. I spent 5 years looking for this and I found it. (YT: #10) BLEACH OST - Shiro Sagisu - BL_93 (Soul Society)) for anyone in my position