Spawners are capable of generating enemies infinitely until destroyed or the number of on-screen enemies has reach its maximum. There are several types of spawners, but each spawner is only capable of spawning enemies of a particular group.
For each type of enemy there is a summoning stone that will summon those enemies. Different summoning stones are different colors to indicate the type of monster they will call. There is also a strong and a weak version of each summoning stone. The weak version is smaller, less intimidating, has fewer HP and can only summon common and uncommon monsters of that type, while the strong summoning stone can call common, uncommon, and rare monsters.
Uncommon and rare monsters can only be summoned one at a time by any individual summoning stone. An uncommon or rare monster summoned by a summoning stone is tied to that stone, and that stone cannot call another monster of that type until that monster is killed. Note that it is possible to have more of those monsters on screen at a time, because the monsters could have spawned separately from the summoning stones, but once you kill those monster the stone cannot spawn a new one to replace it.
Many summoning stones are unique to the Colosseum mode, sometimes employing acid, lasers, or tornadoes.
Mini Summoning Stone
Slayer Edition introduced lower-tier versions of summoning stones. These lower-tier summoning stones are smaller than full-sized summoning stones and only spawn the basic enemy units of any given monster type. For example, a mini summoning stone that summons Demon monsters will only summon Demon Fiends because they are low-tier enemies but will not spawn Demon Imps since they are high-tier enemies.
Bone piles spawn Ghosts and can only be found in Endless mode on Crypt floors. A single bone pile spawns three Ghosts at once and displays a glow immediately before generating them. Aside from being summoned by Liches, bone piles are the only other way Ghosts can appear in the game after the Slayer Edition update (Death Incarnate used to be able to summon Ghosts).
Bone piles are reminiscent of spawners in the original Gauntlet (1985) that have an extremely similar likeness in both aesthetic and function.
Rifts exist as a component of the boss fight in Heart of the Gauntlet. In this fight, Morak uses Sword of Tyrfing to create three rifts at a time that summon enemies from each realm depending on the phase of the battle. Unlike all other types of spawners, rifts are time-based and cannot be destroyed with force.
Some enemies themselves are spawners as they possess the capability to summon other monsters or spawners. These capabilities are limited to bosses and mini-bosses except in the unique case of Corrupted Mummies, who can spawn Bloated Mummies by transforming Risen Mummies into them.