Summon demons is a special ability of Pit Lords, that enables them to summon a stack of dead creature back to life as Demons. Non-living creatures cannot be summoned as Demons. The ability is based on dead stack's health (see calculation), and therefore health altering artifacts Ring of Life, Ring of Vitality, Vial of Lifeblood and Elixir of Life affect the outcome of the summoning.
An individual Pit Lord can summon up to 50 health points worth of Demons. For example, because Demons have 35 hit points can 7 Pit Lords can summon exactly 10 demons. However, total health of Demons summoned this way can not exceed the total health of the dead stack nor can the number of Demons exceed the number of creatures in the dead stack. In other words, the dead stack must have had at least 35 hit points in order for summoning to work. A stack of one Pikeman cannot be resurrected while stack of four will summon one Demon.
In order to use full potential of resurrection, [dead_single_health] shouldn't exceed the health of demon (35). If the stack consisted from imps, optimal count of units is 12.5 times count of pit lords; if the stack contained gogs, optimal count of creatures is ~3.8 times count of pit lords (provided no health increasing artifact is in place).
Strategy – demon herding
Demon herding is a tactic that can be used for Inferno. The basic idea is, that the player deliberately sacrifices imps and gogs in combat in order to summon them back as demons. This is based to the fact that imps and gogs have lower attack to defense ratings than demons, and therefore their hit points are more valuable. You can also herd demons from cheap low level troops from other towns. Additionally, cloning hero's pit lords will grant and extra number of demon summonings. By using the Clone spell or having multiple Pit Lord stacks, it is possible to resurrect the same Demon stack several times during a single battle.