If you know of, or have played, the Disciples (I and II), then you can pretty much just skim this, as the rules are quite alike.
If a bit rambly.
Basically, you have an overland view of the world, in which units move in stacks. There are various places of interest on the map, like cities, ruins to explore, various shops, miscellaneous quest areas. Wandering/barbarian units also appear from time, to say nothing of whoever decided to pick a fight with you (or vice-versa).
Stacks are groups of units containing 8 unit slots and at least one, able bodied, unit. The slots are organized in two rows of four slots each, the "front" and "back" rows. Most units occupy one slot in a stack (so a stack may have at most 8 units). Heavy units occupy two slots, one in the front row, and the back row slot adjacent to it.
Some (but not all) Heavy units are Mounts. They may carry another, non-Heavy unit, and that unit, called a Rider, is considered to be placed in the back row slot occupied by the Mount.
On the overland map, stacks move with the speed of the slowest unit (barring special circumstances).
One such circumstance, arguably, is when carrying bodies around. Carried bodies must occupy stack slots as well; they can be Riders, but they can never be Mounts. There must be at least one living unit in a stack however, and all living units will incur a penalty to their move by a factor of (living unit number)/(total unit number). The adjusted move is used to decide how fast the stack can move.
Living Mounts can carry 1 non-mount corpse without incurring a penalty. A corpse so carried does not count in the calculation of total units in the stack for the purpose of determining movement penalty.
A specific ability of your side is that bodies can be delivered to a city and once there they can be used when popping new units. The XP that the dead unit accumulated can be, in part, acquired by the new unit. How big that part is depends on the archetype of the new unit compared to the archetype of the corpse it uses, choices the new unit makes when using the XPs to level up, but it can never be above 90%. A unit can acquire XP from a corpse only once in its life.
There are four unit archetypes that a new unit can choose to be: Melee, Archer, Archon, Support. Each archetype gets a choice between Might and Magic path at level 2, and a choice between Skill and Force at level 3. Choosing the same options allows a new unit to recover 90% of a corpse's XP. Choosing differently at a step places a 2/3 factor on recovered XP. Example: an Archer that wishes to follow the path of Magic and Force will recover, from a Melee Magic Skill, 40% (90%*(2/3)*(2/3)) of XP.
No other side has this ability. Use it well.
At the beginning of each turn, units receive rations that are drawn from the side's treasury. It costs level*10 Schmuckers to heal 1Hit for a unit of a given level. The players are free to allot the healing as they see fit, within the available resources. A unit might not receive any rations in a turn, but in that case it loses one hit.
Popping and building cities also cost money.
Units that reach 0 hits are dead.
{EDIT: how could I forget this: there is NO maneuvering in combat, ever. Units stay put in the same slots until they flee, die, or win the fight.)
When fighting, stacks engage front row to front row, which are considered adjacent to each other. Melee units are restricted to attack adjacent enemies, which means, enemies located either in enemy stack slots that are adjacent or, if those are empty, the closest enemy front row slot with a unit. If the enemy front row is empty, it "vanishes" and the enemy back row becomes adjacent. Archer units may strike at any one target, regardless of its location. Archon units attack all opponents at once. Support units, usually, provide buffs for allied units.
An exception to the above adjacency rules is for Melee Riders. A Melee rider can attack slots adjacent to the front of its Mount, but cannot be attacked by enemy melee until the mount is dead, in which case the Melee Rider ceases to be a Rider, and is simply a Melee unit placed in the front row slot occupied by the mount.
Combat takes place in turns. During a turn, the Combat score incremented by 1d5 decides the order in which units act. Ties are broken randomly. Higher adjusted combat acts first. Combat scores are between 10 and 90.
When a unit is to act, it may choose from surrender, retreat, attack, stun (halves damage, but a unit brought to 0 hits by a stun is revived as a prisoner, with 10% of hits, after combat ends, if its stack lost, and simply revived to 10% hits otherwise), defend (halves damage after defense until the next choice, but forgoes any other action), wait. Waiting means a unit passes on its chance to act. If, once all units got a choice in a turn, there are waiting units surviving, they get another choice (excluding Wait) in reverse order of adjusted combat.
"Damage" score is independent from combat. Here, damage means the result of a unit acting on its target, and may be damage proper, or some other effect, even a beneficial one. Damage has a source, which can be Matter, Motion, Life (for support abilities), Fate, Mind, Erf. Most units have Matter as a damage source.
If a target is Immune to a damage source, it can never be affected by attacks with that source. If it has a Ward, the first attack in combat that has that source will be absorbed harmlessly. No units have immunities against Fate.
Chance to hit should be self explanatory.
Assuming no immunities and no active wards, the damage dealt is Damage score of attacker*(1-Defender's Defense score/100). Defense scores will always be within 0 and 90.
Some units have a secondary attack. It can only occur if the primary attack hit and was not blocked by immunity or a ward. The second attack also has a chance to hit. In general, the secondary attack inflicts a condition on the target, like Poison or Frostbite, which while not, stat-wise, impressive, bypasses an enemy's defense score.
A unit's defense score is influenced by its natural defense, the items it carries, and the location it fights in- only defenders in cities or ruins benefit from this.
Humanoid units can carry items. Using certain item classes are skills that can be learned on level up. There are, on any humanoid unit, a slot for head gear, for armor, for footwear, two hand item slots, and two potion slots, the latter can only be used for potions (duh). Potions tend to be most useful imbibed before battle, with the notable exception of healing potions.
In general, stacks are not required to have a leader (unlike Disciples). If a stack has a leader however, all its containing units receive a slight boost to combat (+5) and defense scores (+10%), and a more significant bonus to damage (+1). Leadership is an ability that may be picked up on level up.
Leveling up happens when a unit has accumulated enough XPs. Most XPs will probably be received through combat; surviving units of the winning stack receive an equal share of the XP.
On level up, a player unit gets to choose from various skills and abilities (wards, Leadership, item usage, stat improvements, secondary attacks). Levels 2 and 3 bring a choice; to make things easy, I'd rather this choice be announced when you pop the unit and locked in then. It becomes effective on the respective level, ie. the unit morphs into the appropriate type. Units may, in principle, reach any level if they survive long enough.
In general, you can expect levels 2 and 3 to bring fairly massive boosts (relative to the previous stats). Levels 4 and 5 also offer significant improvement. Beyond that, it's a steady trickle.