In folklore, mythology, and various fictional works, there are numerous depictions of powerful demon swords. The concept of a "most powerful" demon sword can vary depending on the specific lore, story, or universe being referenced. Here are a few notable examples of powerful demon swords from different sources:

1. "Soul Edge" (also known as "Inferno") - Soulcalibur Series: Soul Edge is a malevolent sentient sword from the fighting game series Soulcalibur. It possesses immense power, corrupts its wielders, and seeks to devour souls.
2. "Tessaiga" - InuYasha: Tessaiga is a powerful sword wielded by the protagonist of the anime and manga series InuYasha. It was forged from the fang of a powerful demon and possesses the ability to vanquish supernatural creatures.
3. "Rebellion" - Devil May Cry Series: Rebellion is the signature sword of the main character Dante in the Devil May Cry video game series. It is a demonic weapon that can transform and unleash devastating attacks against enemies.
4. "Stormbringer" - Elric Saga: Stormbringer is a black-bladed, soul-devouring sword in the fantasy series by Michael Moorcock. It grants immense power to its wielder but is also cursed, bringing tragedy and draining the life force of those it strikes.
5. "The Sword of Khaine" - Warhammer Fantasy Battles: The Sword of Khaine is a mythical weapon associated with the elven god of war. It is an incredibly powerful sword said to grant extraordinary martial prowess but also drives its wielder to madness.

These examples represent just a few instances of powerful demon swords in different fictional works. The notion of the "most powerful" demon sword can vary depending on the narrative and the specific characteristics assigned to the sword within its respective story or lore.