Similar to Force Fear, horror, insanity, or Torture by Chagrin, summon fear was a dark side Force power conjured through Sith magic, that caused the victim to experience their worst fears to the point of hysteria. It was a very powerful ability, and the duration and intensity varied depending on the caster.
This spell was called upon when the caster moved their fingers in an intricate pattern, likely focusing the Force in a particular way, taking seconds to produce effects. The victim's deepest and most primal fears were made seemingly alive, often taking the form of demons or monsters that relentlessly torment the victim. The longer the spell is held, the more realistic and terrifying the illusions became. To an outside viewer, the victim may seem to suddenly seize and go insane.
Depending on the spell's length, the effects could range greatly. Short durations could result in the victim waking up with only a small recollection of what happened. Longer applications could cause one's physical form to seize itself into unconsciousness, while the mind was virtually destroyed. The body remained effectively comatose, while whatever lasting consciousness was tortured for all eternity. Spells even longer endured could very well result in the victim's death.
Darth Zannah, Dark Lord of the Sith, was a master of this ability. She first used it against the Chiss female Cyndra, causing the woman to immediately go insane and tear her own eyes out. She used it again when dueling Jedi Knight Sarro Xaj on the planet Tython, and again shortly after that, against her own cousin, Darovit. Ten years later, she applied it against her Master, Darth Bane, during their fatal duel; however, Bane was able to conquer his fears long ago and, after a few moments, resisted the spell.
In 33 BBY, in attacking a Black Sun fortress on Ralltiir, Darth Maul may have used the spell on an Iktotchi bodyguard when the latter tried to invade the Zabrak's mind.