The "Odinsword" is a name that has been applied to several Asgardian weapons kept by Odin.
"The Over Sword of Asgard"
One version is also known as the Over Sword of Asgard. It is a special weapon; a giant-sized sword that, if ever unsheathed, it was said, that "the end of the Universe is at hand." One story on the origin of this weapon said that it was created from a cursed Nibelung ring of gold that would bring about the end of Asgard. Greed for the powerful ring had cost many lives, including that of a mortal incarnation of Thor, due to Odin's greed for it. Upon reviving Thor, Odin sacrificed himself to atone for putting Asgard at risk for his greed and split the ring to act as bands to hang himself on the Yggdrasil. But while so hanging, he learned of the coming of the Celestials. He freed himself, and to attempt to contain the ring's curse, he transformed it into the gigantic Odinsword, which he alone could draw safely, since the curse was too powerful for him to purify. Over the years that followed, several enemies of Asgard attempted to draw the Odinsword to destroy Asgard, and the Universe with it, including Mangog. Odin used the sword as a weapon for the Destroyer armor after it absorbed the souls of all of Asgard. Despite its overwhelming power, it proved to be completely ineffective against the Celestials. Thor threw it into the Celestial Arishem the Judge, Leader of the Celestials's Fourth Host, who was unharmed. Arishem analyzed the sword, then proceeded to melt it down, exorcising the Nibelung curse in the process.
"Ragnarok," "the end of all things"
A second version of the Odinsword was revealed during the Serpent's War. During the crisis with Skadi, Odin presented a sword to Thor, claiming that it was the Odinsword and revealing its true name to be "Ragnarok," "the end of all things," prior to the final confrontation with Cul Borson. Odin stated that it was the only weapon that could defeat Cul, since no other weapon could.
- A third possibility for an Odinsword appeared in the World-Eaters storyline. Thor used a giant sword to cut through space/time of the World Tree, but this was never officially called the Odinsword in the comic itself, and only so called later, in one of the pre-issue descriptions.