Isshin Kurosaki

Despite everything, Ichigo knew that he still wouldn't stand a chance in a battle against Aizen, so he needed to do something he'd been afraid to do this whole time.

Entering a void between worlds, Ichigo and his father, Isshin, began their training.

The idea actually came from Isshin himself.

After revealing his identity as a former captain, Isshin told Ichigo about a technique called the Final Getsuga Tenshou.

This was something Zangetsu had apparently been wanting to teach Ichigo this entire time.

The only way to access this technique was to enter a place called the Dangai, a dimensional corridor between the human world and the Soul Society where time moved completely differently.