...a legendary sword forged from the depths of hell. To use it one mustn't be a virgin otherwise it'll kill them with its fire, and those who are able to wield it would have a frighteningly powerful sword at their command. It's supposed to be able to call forth very dark and destructive magic.
— Act VII, Ch.19
The Hellfire's Edge is the darker twin of the equally legendary Archlight's Blade, capable of purely destructive magic. It and the Archlight's Blade are said to be the only weapons capable of killing Jovian & Jacqueline the Gemini, at least according to alpha witch Charlotte.
The sword's appearance is a dark, obsidian blade with glowing crimson runes near the black hilt. It gets its name from its edge being coated in flames.
The sword, which is considered evil, is capable of sending a haunting aura towards anyone close by when it's unsheathed. Those who hold it feel its terrible power, and the stronger the power of the person, or the more terrible their anger, the hotter the blade will burn. It's so powerful, it can wipe out an entire field and several trees with a mere swipe.
As the legend goes, it was forged by demons and only those who have lost their virginity are allowed to hold or even touch the sword. To try otherwise would result in instant death by fire (or perhaps not so instant...).
One exception to the rule above is Ember. Being a witch who is unable to be burned by fire, thanks in part to the magic she inherited from her mother, Alyssa, Ember would technically be able to hold the Hellfire's Edge regardless of whether she was a virgin or not.
Some time after Jovian & Jacqueline were sealed away, the Archlight's Blade and the Hellfire's Edge were forged in the off-chance the Dark Queen of Anarchy was ever freed. The Hellfire's Edge was then entombed into Mount Volrof, but was eventually obtained by Ember, who would later dub herself "The Hellfire Witch" to match her prized sword.
Being a witch who gains her power from fire, the Hellfire's Edge proved to be incredibly dangerous in Ember's possession as it served to consistently kindle her already enormous power, making her energy reserves virtually endless.
In addition to the sword boosting her power, Ember was able to make the sword fight alongside her, where it swung about in midair like a second fighter.
Eventually, the sword ended up being used against Ember when she tried to kill Alyssa out of revenge in Act VII. Sending it flying to it's victim-to-be, the Hellfire's Edge was caught by Kitten, who then used it to fatally strike Ember through the back, ending her life with the very weapon she once wielded.
From that point on, the sword's new master is Triska Raylight (whom she shares with Kitten).