Vissidal is an island that was raised up from the sea floor during the Under Cover of Night. Its name means "Summer Palace" in the Falatacot Language. Vissidal has fauna unique to the island and found nowhere else (though corrupted variants of some can be found on the nearby Dark Isle).
Route
Access to the island was granted upon slaying of the Leviathan and players level 130 and higher can now get flagged via the Gateway to the Deep quest.
In addition to the flagging quest, there are three ways to get to Vissidal (once you are flagged).
Take the Haliana portal in Cragstone (26.2N 46.3E) the follow the shore southeast to a Vissidal portal at 70.2N 65.6E.