Behemoth

"It was the most frightening thing I've seen on the sea. A huge crest of wave and someways I was just thrown over and then the whole ship…it was just gone. I saw it vanish into that great maw and it slammed just. The thing was then just gone, dived below.
But that wasn't the weirdest thing that day. No, that was the folks who picked me up. And the place they took me. These folks, they musta been mermaids or someway. They said something to me I couldn't get. Then one of 'em shoved a jelly fish or something in my mouth and they pulled me under. I can swim ok, gotta be able to do that on the sea, but I can't stay under at all. So I started freakin' out. But I found out pretty quick that I could breath through my mouth with that jellyfish thingy.
They took me in a…boat, I guess, to a…thing. I don't know what it was. Maybe a giant flower bulb, like my granny made me help with? It looked like a bug or something had broken out of it. Then we got closer and that bug hole? Musta been where that thing that at the boat broke out.

The Behemoth is an enormous beast created by bioengineers to ravage enemy navies. But it was impossible to control, so its creators also had to create a containment system for it. They would have killed it, and tried, but it is nearly invulnerable to physical damage.
The behemoth survived the return of magic, as did it's containment facility and the guards operating it. But within days of the Return, the monster escaped. It had abosorbed some of the magic and it was made all the stronger for it. Over the years, the attendants, who had also been bioengineered so they could operate deep underwater without breathing apparatus, repaired the breach and eventually recaught the Behemoth. But this became a pattern over the centuries. Every now and then the creature would escape, the captures would hunt it down and return it to its cage. Thus, stories and legends grew around the behemoth. And while the stories were wild, they were always fairly accurate about the beast that cannot be killed.