He Who Sleeps

"He Who Sleeps" is a pseudonym for a giant dragon who has been sleeping for many years in the plane of Limbo.
In S05E19: Coffin Technicalities and Many Things, Averlyth learns from a sphinx that he is an ancient evil, legendary dragon, he is the counterpart to the queen of the Drakeguard, they are lifelong enemies. There’s no vulnerability or magic weakness.
== Notable Episodes ==
* S05E09: SUPER NOT COOL
* S05E19: Coffin Technicalities and Many Things
*Season 7 in general