The Battle Within is a music from the Plague of Shadows soundtrack. This is the battle theme of the Corrupted Essence, and a slow-paced and dramatic arrangement of Flowers of Antimony as a basis. It is Music Sheet #13 in Plague of Shadows. It can also be listened to in Sound Test after completing the campaign once.
Music Sheet #13 is unlocked by giving Percy 39 Scrap Sheets.
"Very scary. Sometime scariest thing is self!"