I've always loved the music from Sora and Riku vs Xemnas, and wanted to do a similar theme for Tiamat. Woven into the melody are bits and pieces from the original battle theme for Tiamat. This fight takes place after defeating Tiamat for the first time, only to discover that he was merely testing the strength of the protagonist while waiting for coalescence to begin. In coalescence, the victor will absorb the defeated (Tiamat is the darkness split from the protagonist, so they're the same person). This fight is unwinnable and meant to progress the story. Listed below are the songs related to Tiamat in chronological order.

Music © Shawn Badolian

World Eater
At Worlds End
The Battle Against Tiamat
Coalescence - This song
Seraphim
Destiny's Embrace
Fin Fatale - The final battle theme for Tiamat in progress.