Atlantis is a nation based on the imaginary continent from Greek mythology that is said to have sunk beneath the sea, who likes to confuse drunks by riding through the room along with other mythical creatures (such as leprechauns, pixies, unicorns, and flying pink elephants) to make them think they've had too much to drink, as seen in Drunken Party. Humon states that he has been pulling this prank throughout history.
It is implied that Greece is the only one who is aware that Atlantis is an actual entity and not just a hallucination, and is not pleased with how he frequently ruins drunken parties of his with his pranks.