The magical sword in Slavic tales that can cut through anything is the Kladenets, or Sword Kladenets.
Kladenets is a legendary weapon appearing in Russian fairy tales and byliny, the epic poems about heroes such as bogatyrs. It is usually presented as a wondrous sword of exceptional power, allowing its bearer to defeat enemies and overcome obstacles that ordinary weapons could not handle. English translations variously call it a magic sword, hidden sword, or sword of steel.
The weapon’s name is connected by some interpreters with the Russian word klad, meaning a treasure or hoard. That reading fits a recurring story pattern: the sword is hidden inside a wall, beneath a rock, or under a sacred tree, waiting for a worthy hero to discover it. Other philologists dispute this etymology, so “Kladenets” should be understood primarily as a traditional epithet rather than a perfectly settled translation.
Some commentators equate Kladenets with the self-swinging sword mech-samosek, while others distinguish them. That variation explains why magical swords in different Russian tales may share similar abilities without being exactly the same named object.