Undead Whale (5e Creature)
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Gentle giant. The undead whale always attacks at lowest initiative.
Lumbering behemoth. The undead whale takes actions only every second round.
Belly of the beast. Any creature swallowed will suffer 1d4 acid damage per round.
Swallow. Melee Weapon Attack: Hit: 5 (1d10) bludgeoning damage. If the target fails a Dexterity or Wisdom saving throw, they become swallowed by the whale, and are effected by Belly of the beast every turn until they escape it's body.
Rotting corpse. (5/Day) The undead whale can perform a cloud attack to 40' radius either insect plague or stinking cloud, but no more than 3x of each cloud type in any 30 min period [ ie 3 of each cloud type in 30 mins].
The undead/zombie whale is the magical transport / mobile home / lair for a sea hag which can travel as a ship or submarine or be beached as the sea hag desires.
The entire huge creature is disgustingly foul to most senses and liable to cause nausea and similar effects. Typically the blubber flash is bleached and white with the outer skin peeling and cracked. There might also be barnacles and old embedded harpoons.
On an underwater dive the carcass can retain air internally for as long as the sea hag chooses- but cannot be renewed without surfacing if the Sea hag chooses the air can be released but she cannot replace it without re surfacing
The dripping rotting flesh can (if the DM chooses) contain various unpleasant small creatures - crabs, rot grubs , green slime etc
There will be 2d6 sea gulls (use raven or krowk stats)- either alive or undead which can be used as spies or for harassing defense.
The stomach of the whale is a typical sea hag lair with sea weed bed and appropriate beach combed furniture, with other organs used as storage sacks.
Undead whales are the size and hit points of a kraken or tarrasque and have the potential to destroy a party quickly at the whim of the sea hag - and are more designed to be encountered beached as a horrific form of sea hag lair than an active creature - although it can be used to attack characters or break up other ships the intent is more to give the sea hag a form of ship suited to her nature. It could be met on a lonely beach or perhaps encountered by ship wrecked PC's adrift at sea with the sea hag offering 'rescue' in return for a special favor.