Shugoryuu

The Shugoryuu (守護竜 lit. Guardian Dragon) are a group of Shinigami and Humans who have gained the ability to morph into dragons.

Becoming a Shugoryuu
Shugoryuu are rare beings because of how they are created. To become one a person must ingest an Iwa no Ryuu (岩竜 lit. Rock of the Dragons), a rare rock found only in Riku no Hatsu. Once ingested, at 8 AM, the time period other Shugoryuu call Tatsunokoku (辰の刻 lit. Hour of the Dragon), the ingestor passes out. During this time their body slowly turns for the first time into a dragon.

Once they awaken they are trapped in dragon form for at least an hour after regaining consciousness. During this time period, they can at time lose their sentience and will normally rampage out of control. After this, they must learn to dominate their new powers or else be consumed by it forever.