In Norse mythology, Mjölnir (Old Norse: Mjǫllnir - “Lightning” or “That which smashes”) is the hammer of Thor, the god of thunder and lightning. Forged by the dwarven brothers Brokkr and Eitri, Mjölnir is the most feared weapon in the Nine Realms, capable of leveling entire mountains with only one hit. Mjölnir was also a symbol of both fertility and destruction, and was thought to possess regenerative healing powers.
When thrown, Mjölnir would return to Thor's after hitting its target.
"... He [Thor] would be able to strike as firmly as he wanted, whatever his aim, and the hammer would never fail, and if he threw it at something, it would never miss and never fly so far from his hand that it would not find its way back, and when he wanted, it would be so small that it could be carried inside his tunic."
- Orchard, Andy. Norse Myth and Legend. London: Cassell, 2002. p.255