Pantala

The Lost Continent is a location mentioned in Darkstalker (Legends) and Runaway. It is mentioned that Clearsight, as a young dragonet, dreams about exploring and finding it but is dissuaded, seeing that the path where she left to find it ended in her dying somewhere over the sea because that path was so unlikely. When she is at Darkstalker's home, she notices he has a scroll titled "Myths of the Lost Continent". Later, after betraying and imprisoning Darkstalker, Clearsight might've decided to go and find it. It is shown in Clearsight's visions to have animals she hadn't seen before, as well as colorful dragons of unknown tribes as well as a new love and possibly different dragonets. It is possible that they may appear in Darkness of Dragons.

It is possible the Lost Continent is the NightWing's old island home. This would make sense, if Clearsight decided to explore it.

Also, in the post-epilogue of Darkstalker (Legends), it states:

"Post-Epilogue - Two Thousand Years Later

Centuries later, as the dragon planet spun through space, a comet passed by, close enough to shine like a fourth moon in Pyrrhia's sky.

Close enough to change the tides and shake the continents. As earthquakes rumbled through the ground, long-buried rocks shifted that had been in place for thousands of years.

Deep underground, in the darkness, copper wires snapped.

And a dragon awoke..."

— US Hardcover - Page 366

As the word "continents" is plural, this indicates the Lost Continent does, in fact, exist.