I have been running a game of Dark Ages: Mage. Due to certain events, there is going to be a battle between two armies. How is the result of such a fight decided, according to RAW?
For a Dark Ages: Mage game, you'll find the rules for mass combat — fights between armies — in the Dark Ages Storyteller's Companion, under the section marked "Blood & Tears." It's oriented with an eye toward vampire combatants, rather than mages, but you'll find it easy to modify the elements you need.
To answer your question directly, the winner of the fight is decided in one of three ways, as per page 69.
- Storyteller fiat.
- Use the actions of the player characters to determine the result of the battle, in full or in part.
The army leader with the greatest score in the Blood & Tears subsystem or the most successes accrued wins the day.
- Leaders roll Perception + Leadership to achieve tactical objectives and add margin of successes to their troops' rolls.
- PCs in combat roll their attack pools vs. a difficulty of 6; each success reflects a dead enemy. ('Enemy' here means an unnamed NPC soldier, not a named adversary.)
- The use of lethal Disciplines (or, in this case, magick) leads to two dead foes for each dot in the effect used.
- Counterattacks are reflected by rolling 5 dice, difficulty 6. If 5 successes are accrued, the PC is incapacitated. Otherwise, roll two damage dice for each success; this is lethal damage. Every three damage successes translate into one aggravated damage.