The Real Knife is a weapon item found in Asgore's Home. Plot-wise, it is the Worn Dagger as seen through the lens of a Genocide Route. Its defensive counterpart is The Locket. It is the Genocide Route weapon item associated with the first human, and is implied to belong to them.
- If the Real Knife is hacked into the inventory on a non-Genocide Route and is equipped, most monsters can be spared on the first turn (even most non-spareable monsters).
- Despite being the strongest weapon in the game, it never sees any real use without hacking, as Sans, the last enemy of a Genocide Route, has only 1 HP. The same applies to The Locket, the strongest armor in the game (as the last enemy can only deal 1 damage per hit).
- The Real Knife could possibly be a reference to Mother 3's Real Bat, since both weapons are found before the final battle and wielded by someone before the protagonist.
- Prior to the full release of Undertale, Toby Fox had denied the existence of the Real Knife.
- Where are the knives. - Narration when checking Toriel's kitchen drawer during the Genocide Route.
- "Apparently some person on TVTropes started a rumor about this. There’s no such thing. This is a game for all ages, after all." - undertale on Tumblr, June 09, 2013.