Ysmir is the Nordic aspect of Talos. According to the Greybeards, the name "Ysmir" is also a title which means "Dragon of the North." It was given to the Emperor Tiber Septim, who withstood the power of the Greybeards' Thu'um, they recognized him as a Dragonborn and gave him the title.
Ysmir the deity is the Nordic aspect of the Imperial God known as Talos, after he had been called to High Hrothgar by the Greybeards to conquer Tamriel. On his death, he became Talos in the Imperial Pantheon, and Ysmir in the Nordic one.
Ysmir Wulfharth was a Nordic King during the First Era. His Thu'um was said to have been greatly powerful. Originally, Wulfharth thought he was called by the Greybeards when they were calling for Talos. But when he arrived, he was destroyed by their voices, as he was not the one destined to unite Tamriel.
The Last Dragonborn
During the events that unfold in The Elder Scrolls V: Skyrim, after completing "The Horn of Jurgen Windcaller," the Greybeards greet the Last Dragonborn in the dragon tongue with the words: "Lingrah krosis saraan Strundu'ul, voth nid balaan klov praan nau. Naal Thu'umu, mu ofan nii nu, Dovahkiin, naal suleyk do Kaan, naal suleyk do Shor, ahrk naal suleyk do Atmorasewuth. Meyz nu Ysmir, Dovahsebrom. Dahmaan daar rok.". This can be translated by: "Long has the Stormcrown languished, with no worthy brow to sit upon. By our breath we bestow it now to you in the name of Kyne, in the name of Shor, and in the name of Atmora of Old. You are Ysmir now, the Dragon of the North, hearken to it."