Dweller in the Deep

The Dweller in the Deep is a legendary aquatic monstrosity that rises once every 13 years to cause chaos and destruction along the coast.  The juvenile is approximately 120 ft. in diameter, has four large tentacles and a large central maw.  It is also capable of strong long-range psychic attacks. If not driven back, entire towns have been lost as it hungrily tears them from the coast.  When the time is right and its return is near, it can be summoned for battle by singing the following sea shanty:
In the night on sails of old Atop the winds of water cold Deep within the dark abyss Lies a beast asleep in bliss When storms abound and lightning calls The sea she boils and darkness falls It wakes, it roars, writhes and creeps While rising from the briny deep Woe betide all creatures be, Hear them scream, watch them flee! The great consumer, the eater of worlds It searches, it grasps, maw unfurled For those who dare to sail on seas Ruled by a beast, that we name thee The Dweller of the Deep!
According to legend there is only ever one Dweller at a time.  When it rose most recently in 1491, the Monster Squad and Captain Tr'Alik discovered that the elder Dweller had died some time recently, and that the juvenile had risen.  The elder Dweller had been killed in a battle with nine Mind Flayers and some other larger creature.