The Darkest Hour (2006)

From the introduction:

Welcome to the final part of the Drow War campaign, The Drow War, Book Three: The Darkest Hour.

This adventure is epic in both senses of the word familiar to d20 enthusiasts. It takes the action out into the manifold planes of existence, pitting the Player Characters against no less a threat than the extinction of all life and the final triumph of darkness- stakes far beyond anything faced before. The adventuring takes place on a cosmic scale, with allies and adversaries to match.

The Player Characters move beyond the standard 20 levels of regular d20 and enter the realms of the epic rules. They come into possession of astounding new magical items, learn feats and spells that outshine the heroes of legend and face down foes that cause entire civilisations to tremble.

The action begins on the world of Ashfar, where the peace forged in the aftermath of the Player Characters’ apparent victory at the end of the last book has now been broken. Something is causing the barriers between worlds to collapse, smashing plane into plane, opening rifts through which atrocious fiends can step into the ordinary world and cause chaos. No hero can let this happen without acting against it, and on this world and others, armour is donned and swords are drawn.