not too sure if this is his theme specifically, but i can only recall hearing it during the first phase of the second fight against him. he actually has multiple themes.

FINAL FANTASY XVI ULTIMA THEME

Final Fantasy XVI Original Soundtrack Ultimate Edition

Away

No Risk, No Reward

'Neath the Pall

Chopin Etude "Océan" op.25 n°12 (orchestral version)

Epic orchestral fantasy music for DnD, tabletop RPGs, and immersive storytelling.
Cinematic symphonic tracks inspired by myth, magic, heroes, battles, and ancient worlds. Ideal for D&D sessions, fantasy campaigns, worldbuilding, writing, gaming, and focus.
For fans of epic orchestral, cinematic fantasy, RPG background music, and soundtrack-style compositions.

Something Scary

07. Stand Your Ground -Final Fantasy XV OST

47. Veiled in Black (Arrangement) -Final Fantasy XV OST

Magi Original Soundtrack ~Up to the volume of Balbad~ by Sagisu Shirou

MYM 'Devin' - Sagisu Shirou

Notre puissance - Sagisu Shirou

Danse bizarre 03 - Sagisu Shirou

Notre puissance - Sagisu Shirou

Offense Armor