Upon use, Lemegeton will grant a random passive item as an orbital familiar. The item summoned has a 25% chance to be from the current room's Item Pool, or from a random item pool otherwise. The item must have the 'summonable' item tag in order to be generated.
These familiars can block up to 4 shots and/or get hit by 2 explosions before being destroyed. When the familiar is destroyed, the item it granted is lost.
The Lemegeton is another name for the Lesser Key of Solomon, an anonymous grimoire focused on demonology that was compiled in the 17th century.
After using Sacrificial Altar or Genesis, Isaac keeps the passive effects of each wisp until the next use of Lemegeton, which erases all sacrificed passive items.
If a run is exited and resumed while Isaac has item wisps generated from Lemegeton, all wisps will be condensed into the middle clockwise ring, even if this would result in more wisps on that ring than normally allowed. This also allows further wisps to be generated on the inner and outer rings.