Into The Dark is a set of free wargames rules for mass combat in the Dark Ages / Medieval era. From the author’s introduction:
I’m aware that there are plenty of rule sets on the market for the medieval period. However, many of those rules are actually designed for skirmish fighting. Even some of the better known rules for mass combat are really just skirmish rules done on a giant scale. Into The Dark is designed for battles between armies, similar to those found in history or heroic literature. Large scale battles ‘look’ like large scale battles, and yet can still be fought to a conclusion in a reasonable amount of time.
These rules were originally designed for 25mm/30mm figures, although they can easily be used for 15mm by dividing all ranges in half, or by using centimetres instead of inches. Don’t be put off by the size of the rules, they actually play very fast and easy. Try only the basic rules until you are comfortable with play, then add whatever optional rules you like. The section on game preparation takes up a lot of room, but is only needed when first creating your army. In fact you could skip it if you’re just checking out how the rules work. Come back to it only if you decide to give them a try.