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 have 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.
Clearsight traveled to the Lost Continent alone, not followed by any other dragon.
Tui has mentioned in an interview that she may use Clearsight's journey in the Lost Continent for the third arc, and it will feature new, unheard of tribes. Possibilities for these may be StormWings (the tribe Tui said she would be in) and GrassWings (the original eighth tribe that Tui removed by the final draft of TDP).
Also, in the post-epilogue of Darkstalker (Legends), it states:
As the word "continents" is plural, this indicates the Lost Continent does, in fact, exist.