Sgaeyl

Xaden Riorson's navy-blue Daggertail, mated to Tairn.

Sgaeyl is the navy-blue Daggertail dragon bonded to Xaden Riorson, and his shadow-wielding signet comes through her. She is mated to Tairn, Violet Sorrengail's dragon, which lets Violet talk to her.

Quick Facts

Breed: Navy-blue Daggertail
Rider: Xaden Riorson
Mate: Tairn

Facts from the source article.

Bond with Xaden

Xaden Riorson's power to wield shadows comes through his bond with Sgaeyl. In Fourth Wing he uses those shadows to kill the six cadets who break in to murder Violet Sorrengail.

Sgaeyl and Tairn

Sgaeyl is mated to Tairn, Violet's dragon. At a meeting with the dragons Violet learns she can talk to Sgaeyl because of her own bond with Tairn, and Violet and Xaden find they can speak telepathically through the two dragons. In Iron Flame the dragons' bond lets Xaden, posted at Samara, and Violet see each other every week.

First rider

Iron Flame reveals that Sgaeyl was first bonded to Xaden's grandfather. That earlier bond is why Xaden has a second signet, inntinnsic, which lets him sense people's intentions.