Rivendark is a black dragon that flies over the Vortex Pinnacle. Be very careful around Rivendark — if he detects you he will spit a fireball at you. (~3000 damage)
As of patch 2.4.2, the dragons in Blade’s Edge Mountains will no longer instantly knock you off your flying mount.
Rivendark can be called down at any time by opening Rivendark's Egg at Rivendark's Perch (requires 35 Apexis Shards. When summoned, he lands and attacks the party.
Rivendark drops a variety of items, usually including a random rare item, and also an Onyx Scale of Rivendark.