A Magical Society Aggressive Ecology: The Undead Leviathan (5e)
What is an aggressive ecology? It's a new type of challenge for your players¸ a monster with many faces¸ a web of interrelated challenges that can together form a complex encounter-in-depth. It is a world-building device based upon a magical interrelationship between disparate¸ yet connected creatures. It is¸ in short¸ something not seen before and something not quickly forgotten.
There are few aggressive ecologies found in the deep oceans¸ and only one centered upon the rotting carcass of a fallen whale. A whale fall is an unusual event in and of itself¸ but occasionally these massive bounties of detrital material land in places that simply cannot sustain natural life of any sort: a negative energy sink. These strange negative energies infuse a whale carcass with unholy power giving unlife¸ mobility¸ and malevolence-creating an undead leviathan. But worse¸ after enough time¸ the undead leviathan rises to torment the land!