The movie will actually cover the Holy Kingdom arc, which is about a war with a different kingdom.

The majority of the Re-Estize Kingdom's army was wiped out at the end of the third season, which included most of their commanders and ranking officers. What remained followed Prince Zanac, along with a ragtag force of drafted civilians, in a desperate attempt to fight off Ainz's forces - disgusted by the nobles murdering Zanac in an attempt to curry favor with him, Ainz' interest in the battle faded, and he left the rest to the guardians, instructing Cocytus and Mare to mercilessly crush the ramshackle army, sparing no one. And that's that.