This plugin creates a water monster. The monster can only be seen by the depression it makes in the water and its splashing. It is invincible, but touching it causes damage to players and mobs.
There is only one monster per server, which is always located within 100 blocks of a player (it despawns if it leaves this range). If the monster has despawned, then it will respawn nearby another player.
This plugin has been tested with build 1145 in addition to 1060, so it will most likely function in 1.8 as well (further testing to come when a RB is available for 1.8).
Note: Credit for the idea of a water monster comes from Frictional Games's Amnesia: The Dark Descent