A struggling rock band must fight through petty rivalries, a dickhead manager and the zombie apocalypse, saving their band and the world on their way to the top.

No need for ritual in this instance, zombies are a trope, best to jump into the action - how the good guys choose to fight them. The uniqueness of the action the main characters take to fight the zombies in a zombie flick is what will make it interesting. All the rest of the stuff; obnoxious managers, love interest and inner group tensions are a given and need not be elaborated on in the logline.

My try:
After a zombie apocalypse a rock band discover their unique sound kills zombies they must now save humanity while climbing to the top of the charts.

26 words.

The other reviewers are correct. The core idea seems to be "When the Zombie Apocalypse destroys their world, an aging rock band must find their own unique sound to save the world." or something similar.

What Moses99 said.?The logline needs a unifying framework in terms of a singular objective goal.