Currently, XP playes a major role in endgame- The added health and stats makes units a lot stronger, and is important for champions to be useful.
As the only decent way to gain XP so far is battle, it led me to this spell.
Name: Dark Simulacrum
Target: Caster's Army
Cost: 100 mana
Available via: A researched tech.
Description: A simulacrum of the army appears and attacks it immediately (without the caster). Defending army has an initiative bonus.
This spell is used for training your own troops- giving them real combat, your troops has two advantages: While defending, it has initiative bonus and all of the city's bonuses (if inside one). The second is that the caster not being replicated, giving them a numerical advantage.
Note: Dead units stay dead, they are not illusions in that regard.