From publisher blurb:
Mass Combat Rules for D&D. The D&D combat rules in the Player’s Handbook are designed to model conflict between small groups—an adventuring party of perhaps three to six characters against monster groups that rarely exceed a dozen creatures. Combat on this scale keeps the focus squarely on the adventurers.
In some D&D campaigns, though, the story might hinge on battles involving dozens or hundreds of monsters and warriors. In this second installment of Unearthed Arcana, we build on the standard combat rules to model conflict on a much larger scale, allowing players and DMs to control whole armies. At the same time, these rules for mass combat allow individual adventurers to lead an army’s charge against an enemy regiment, rally dispirited soldiers to rejoin the fray, or defeat powerful enemy monsters or leaders.