The whale explains he's trapped there, guarding a treasure in the wall behind him. The treasure will be theirs if they manage to help.

Spirit Exorcism is basically focusing energy towards the spirit you want to free and attaching a wish to let them go. There are multiple ways for this, so Levi would need to trust his instincts on what to do and think while focusing on his magic.

But the Whale is willing to give it a try - if it doesn't work, it won't harm either of them.