The Republic at the height of the clone wars vs Craftworld Eldar (40k)

Craftworlds Ulthwe, Beil-Tan, Alaitoc, Iyanden and Saim Hann along with all their military assets are transported to the outer rim of the galaxy far far away. For whatever reason, they decide they should attack the most powerful faction that currently inhabits this new galaxy.

Meanwhile the Sith Lord Darth Sidious has been slain by Anakin Skywalker and the republic is moping up the last remnants of separatist resistance. They are blissfully unaware of this new threat to them.