The blood war was a key concept in the 2nd-edition AD&D campaign setting Planescape, pitting the Chaotic Evil demons against the Lawful Evil devils in an eternal war throughout the lower planes. The war wasn’t constrained to just demons and devils, as nearly every other faction participated in it to some degree - Yugoloth mercenaries fought on both sides, celestials took up arms when the war threatened the upper planes, and even the mortal inhabitants of the neutral-ground city of Sigil were swept up in the machinations of the war.
The Blood War miniatures expansion brings these combatants into your game. In addition to many long-awaited demons and devils, this set draws in dozens of planar-themed concepts, including astral raiders, Sigil inhabitants, and outsiders from all around the Great Wheel. Of course, it isn’t all about the planes, because the set fills out a few more notable encounters for the Red Hand of Doom adventure, provides new uniques from both the Forgotten Realms and Eberron, and includes a number of classic monsters and characters.