It was like a survival horror game thing.
Basically for no explained reason you could beath underwater forever.
You were in a huge closed tank with caves on the sides, and you were on two teams.
One team were hunters helping Leviathan to find and kill members of the constuction team, who had to find a certain amount of materials for a stockpile to win.
The hunters weren't allowed to touch people who were being constructive at the time (like moving materials or opening new routes etc), only the people who were hiding like cowards where Leviathan couldn't get.
But they could point them out to Leviathan who could kill you at any time.
And he was a giant long semi-eel like bodied fish with 6 tentacle stalked eyes and a bunch of tentacle graspers on his head so he could look into and reach into all areas of the map.
So basically you had to sneak around finding these fairly sized rocks of material and put them all in this stockpile while avoiding certain death.