In Judgment, Summon Imps is a useful early game Ritual. It is cast via a Dark Altar.
As its name suggests, the summon imp ritual summons between 8 and 16 imps as soon as the casting period is complete. The imps will spawn in proximity to the altar, so be careful and be ready to defend the caster and your base as soon as possible. 16 imps beyond your base defenses can be overwhelming even for a moderately developed base, so do not cast this lightly.
This ritual is a quick and dirty way to replenish your sulfur inventory, as well as an easy method to gain some experience.