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One of the Ancient Dragons.
The as-of-yet unnamed "Deep Sea dragon" is an elder dragon from the depths of a sea with the power to create tentacled creatures from the water. Through its powers, it has been creating servants from every lake and river around its location. Its current whereabouts and all other details are currently unknown.
It is not to be confused with Zhaitan, which also awoke in the sea, but has a different kind of power and minions entirely.
Info taken from Official Guild Wars 2 wiki.