Tiddalik

He woke thirsty and drank, and he did not stop, and now every river, billabong and soak in the country is dry and he is the size of a hill. The animals came and reasoned with him and got nothing. They threatened him and got less. What finally worked was the eel, who danced so absurdly that Tiddalik laughed, and the water came back out of him all at once, and it is still the only thing that has ever worked.

Settle: bludgeoning, melee 20 ft, +4 to hit, 3d6+3, and mostly he is simply in the way rather than attacking. Draw It In: 30 ft, save Con Tough, fail save roll 3d6+3 as he pulls the moisture out of everything nearby, cooldown 3 rounds. Waterskins in the area are emptied, no save.

Too Full to Kill: he cannot be reduced to 0 HP by any means, from any source. Damage stops at 1 HP and goes no further. He can be hurt, exhausted and driven back and he cannot be killed while he holds the water, and the DN should let the party watch their best round achieve nothing. He does not pursue and does not need to. Make Him Laugh: this is the only ending. Something genuinely, physically ridiculous - and the players must actually describe it - releases the water, ends the encounter, fills the rivers and leaves an ordinary frog sitting in the mud.