Fafner

Here's my take on Fafner (or Fafnir, for the original character of Norse mythology), the Giant/Jötunn who, after killing his brother Fasolt out of greed for the treasure of Alberich (the King of the Nibelungs/Dwarves), transformed into a dragon, in Richard Wagner's cycle "the Ring of the Nibelung". As a dragon, he spent much of his time guarding his treasure in his cave, Neidhölh, blowing poison around him, before being challenged and slayed by the hero Siegfried, Wotan's grandson. Then, by bathing in the dragon's blood, the demi-god became practically invincible. In Wagner's operas, Fafner willingly turned into a dragon thanks to the Tarnhelm, another magic artefact forged in the Rhinegold under the orders of Alberich, the first being the famous Ring.

In earlier Germanic sagas, Siegfried still fights a dragon and becomes almost invulnerable by bathing in its blood, but the monster doesn't guard the Nibelungs' treasure. In one version, the dragon is named Regin (a reference to Regin, brother of the original Fafnir), and is not originally a giant, and in another, it has no name.

Finally, in the original Norse story, Fafnir is a Dvergar/Dwarf, who, after killing his father Hreidmar and expelling his brother Regin, again out of greed for the treasure and the Ring of Andvari (the main inspiration for Wagner's Alberich), transformed into a dragon before settling in a cave to keep his gold. It is not known if the transformation was a consequence of the Ring's curse or Fafnir's choice, but I personally prefer the first option ! Anyway, he shared the same fate as in his later versions, dying at the hands of Sigurd (the original Siegfried).