Ahsa

The Leviathan is an enormous serpentine creature living in the methane oceans of Titan. Records of its existence date back to the Golden Age, as its presence registered on subsurface scanning equipment. It can be seen through the windows surrounding the New Pacific Arcology's CPU core in the Festering Halls during the missions Utopia and Ace in The Hole. The Ghost mentions that the arcology's sensors can detect a large object moving under The Rig, and although the sensors can't determine what it is, it is likely the same creature. Guardians have also observed serpentine, finned creatures breaching the surface of Titan's oceans, which are likely the Leviathan or other life-forms native to Titan.

During Season of Arrivals, the Drifter makes reference to "the long boy in the soup" when speaking to Eris Morn, likely referring to the sea monster on Titan. Sloane can also be overheard in Siren's Watch talking about something below the waves, claiming that it wasn't her imagination.

Trivia

 * Before Titan was moved to the Destiny Content Vault, if one went beyond the normally player-accessible areas of Titan and managed to see below the surface of the ocean, they could see . These are likely intended to be the "humps" of the sea serpent breaching the waves, as normally only the uppermost part of the wheels would be visible.
 * Many fans theories stated that this creature might be one of the three remaining Worm Gods, although it is unknown if it is Yul, Eir or Ur. Also, it is suggested that this creature is the late Taken King Oryx's fully developed Worm, as it was unknown what became of it after Oryx's demise. All this, however, is unconfirmed.
 * Soteria mentioned that a "enemy of the Witness is locked up on Titan". It suggest that this being is an adversary of the Witness, the why of it, is currently unknown.
 * An gargantuan eye was seen on the promotional image of the Season of the Deep, being behind the Guardians. This eye is very likely to belong to this creature.

List of appearances

 * Destiny 2
 * Season of Arrivals