Due to their mixed fey–devil ancestry, all forlarren displayed a somewhat bipolar and temperamental personality. Their fey ancestry drove forlarren to act in ways seen as self-destructive and once they reached a breaking point through such behavior the diabolical side of their personality would assert itself.
Forlarren dominated by their diabolical mindset were known to be rather cunning, scheming, and vicious foes. They often sought to dominate or mislead weaker creatures. Once whatever diabolical schemes they had were foiled, a forlarren's fey nature would reassert dominance over their mind.
Sometimes killing an opponent could cause this shift in mental dominance, with the forlarren displaying great remorse for its actions and offering its services to the survivors as recompense. But eventually, their diabolical mindset would reassert itself and the forlarren would leave such a group, attacking them on sight the next time they met.
Forlarrens were innately capable of casting spells and had some resistance to magic. They were generally known to cast the spells aid, expeditious retreat, false life, heal, heat metal, minor illusion, misty step, mirror image, prestidigitation, and Tasha's hideous laughter. But out of all the spells that they knew, heat metal was the one most frequently used.
Forlarren were typically known to attack up close with their fists, claws, and in the case of armored opponents their signature heat metal spell. When they used this spell, forlarren were often quite difficult to separate from their armored victims. With some maintaining contact until either they or their victim was killed.
The exact origin of the forlarren was uncertain; there were two conflicting legends, though both agreed that a devil was involved in their conception. The first legend claimed that the forlarren were descended from the offspring of a good nymph enslaved to an unspecified type of greater devil.
The second story claimed that when the devils of Avernus mounted an expedition into the Feywild, the archduke Fierna tried to court an archfey, with hopes of seducing them into relinquishing their soul. Her efforts ultimately went nowhere, but a group of the lord's satyrs had become infatuated with her. She was unable to claim their souls, for they were bound to their archfey, but Fierna still desired some form of gain from her time spent on the lord. Thus, Fierna decided to corrupt the satyr with her infernal powers and transformed them into what became known as the forlarren.
Within the Feywild, several heroic tragedies and ballads were written about forlarren. These depicted them as valiant figures, but whose diabolical personalities emerged at the most inopportune moments and led to treachery for their allies.
The description of the forlarren's ancestry and behavior in the 1st-edition Fiend Folio notably bears a striking resemblance to "The Fiend," a creature from the first installment of the British gaming magazine White Dwarf's "Fiend Factory" feature.
- Mike Mearls, Bart Carroll, Bill Benham (December 2019). Mordenkainen's Fiendish Folio, Volume 1: Monsters Malevolent and Benign. (Wizards of the Coast), p. 9.
- Don Turnbull (1981). Fiend Folio. (TSR Hobbies), pp. 39–40. ISBN 0-9356-9621-0.
- Jennell Jaquays (1988). The Savage Frontier. (TSR, Inc), p. 15. ISBN 0-88038-593-6.