Aizen Sentiment

The Aizen Sentiment (藍染の思念, Aizen no Shinen) is a secret boss featured in Kingdom Hearts: Arcana. It is the imprint of Sōsuke Aizen's soul left behind as a challenge for Nathaniel Koroshiya.

Requirements
Aizen's Sentiment Portal can be found in the Tranquil Forest of Residential Towers in the Central 46 Compound in Soul Society, where Aizen runs Momo Hinamori through with his Zanpakutō and Tōshirō Hitsugaya confronts him. The Portal leads to a ruined part of Karakura Town, where the "Battle for Fake Karakura Town" took place and Aizen merged with the Hōgyoku. This is only available in Nathaniel's story.