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A hot blade is an edged weapon that somehow heats up, adding the thermal energy of its temperature to the kinetic energy of its blow to achieve Absurd Cutting Power. When active, it will usually glow visibly (without losing any of its structural integrity, somehow) because Power Glows.
Compare Vibroweapon, which uses extremely rapid vibrations instead of heat to enhance its cutting power. A more advanced weapon is the Laser Blade, where the blade is made of Pure Energy instead of just adding (heat) energy to a physical blade. Hot blades are more common in a Science Fiction setting. Its more Fantasy-oriented counterpart is the Flaming Sword; a hot blade retains a touch of Kill It with Fire while requiring less Willing Suspension of Disbelief.
Usually used with swords, but other bladed weapons are also seen occasionally. A subtrope of Absurd Cutting Power. Unrelated to the idiom "like a hot knife through butter"... though hot blades may, indeed, cut through things in such a manner.
- In the Gundam franchise, the next step down from Energy Weapons like beam sabers is heat weapons, the most iconic of which is the Zaku II's heat hawk. Another well-known example is Gundam Sandrock's heat shotels, which become ludicrously huge in Endless Waltz.
- In the battle at the sea in Berserk, Schierke's fire magic was used to turn Guts' Dragon Slayer into one of these.
- Bleach Filler Villain Shusuke Amagai's Zankpaktou has this when its hilt spouts flame.
- In One Piece Enel does a variation by using his electric powers to turn his trident into molten metal in order to burn Luffy.
- Yaiba has Gold, who can apparently heat up his gargantuan Sinister Scimitar somehow. However he does this once.
- Optimus Prime from the Transformers movies uses some of these to cut Decepticon's heads clean off.
- In the first Pirates of the Caribbean film, after Will uses his sword to jam the door of the smithy and cut off Jack's escape route, he grabs a replacement out of the forge. It looks impressively glowy but doesn't last very long; fortunately there are plenty more on the racks.
- Lightsabers are hot enough to cauterize wounds as they inflict them. Which leads to a bit of Fridge Logic as to why the wielder doesn't receive first-degree burns just from holding them.
- Many Warhammer swords (at least in the way they are visually depicted), both Warhammer Fantasy (Archaon) and Warhammer 40000 (THE GOD-EMPEROR OF MANKIND), even when some flames might flicker depending on the artist, verging into Flaming Sword territory.
- Mutant Chronicles.
- Call of Cthulhu supplement Cthulhu Companion, adventure "Valley of the Four Shrines". The East shrine is dedicated to Cthugha, a deity made of fire. A sword found in a secret room can be made to glow yellow-hot, which will do damage to creatures vulnerable to fire.
- The Heated Cosmic Knife from the Dead Money expansion of Fallout: New Vegas is this. It's an indestructible Absurdly Sharp Blade that has been heated with a heating pad and apparently retains its heat for a ridiculous amount of time, giving it even more destroying power.
- One of the bosses from Chrono Cross uses a Steampunk version (a Nigh Invulnerable, Humongous Mecha), but this is technically a fantasy setting.
- The (in)famous Pyrotechnic Axes in Arcanum presumably work this way.
- Strider Hiryu's Cypher uses plasma to achieve this effect.
- Nero's Red Queen from Devil May Cry 4 has a special mechanism inside it; the sword has a gasoline fuel inside, and the sword can be revved; when doing so, it sprays out the fuel to the blade as well as igniting it, making the blade glow hot red.
- Though they are not technically swords, the hooks thrown by Spider Splicers in the Bioshock games are glowing red hot. They are probably heated by plasmid use on the part of the splicers.
- The Thermoblades in X-COM 2 are a combination of Hot Blade and Vibroweapon, and are pretty effective against Demonic Crabs. 
- The Omni-blade in Mass Effect 3, the hologram is so the user doesn't accidentally burn themself.
- ↑ Ok, they are actually lobsters.