Intervention armies cannot be called unless you are at war with the faction performing the ritual. Intervention armies come in three tiers (costing 2000, 5000, or 10000 money), where the more you spend the stronger the army is.
Intervention armies can only be called once per Ritual. If multiple factions are performing Rituals at the same time, you can send one army to each of them.
The Intervention army will spawn close to one of the ritual sites of the target faction and proceed to attack. Intervention armies are similar to a Beastmen Bray-Herds and cannot be directly controlled but can be given a target.