Gargantuan undead, chaotic evil
Armor Class 20 (natural armour)
Hit Points 297 (18d20 + 108)
Speed 20 ft., swim 40 ft.
Saving Throws Str +13, Dex +7, Wis +8, Cha +7
Skills Perception +6
Damage Immunities necrotic, poison; bludgeoning, piercing, and slashing from nonmagical attacks
Condition Immunities charmed, exhaustion, frightened, poisoned
Senses darkvision 120 ft., passive Perception 16
Languages Abyssal, Common
Challenge 16 (15,000 XP)
Bellow (3/Day). Should the battle turn against it, the monster can emit a frightening, unearthly cry as a bonus action, audible out to 100 feet. All creatures that hear the cry must succeed on a DC 14 Wisdom saving throw or be frightened.
Multiattack. The monster makes one bite and one tail attack.
Bite. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 9 (2d8) necrotic damage.
Tail. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage plus 9 (2d8) necrotic damage.
Breath Weapon (Recharge 5-6). The monster exhales a highly toxic cloud of sickly yellow vapor that is 40 feet long and 20 feet wide and high, producing the same effect as that of the spell cloudkill with a save DC of 17. The breath lingers in the air, moving slowly with the breeze and sinking into depressions for four rounds before dissipating.
The undead lake monster is a rare sight. It is an ancient, gargantuan, undead water serpent with grayish-green skin and a huge mouth lined with needle-sharp teeth. Its scaly hide is reminiscent of the skin of a zombie, rotten-smelling and marked with rents and gaps though which pale white bones show. When swimming on the surface, it often appears to be a head followed by a series of rounded humps.
This monster never leaves its lake, but it will attack any creature that comes close to the shore. Before combat has been announced, if the monster has seen intruders near the lake's shore it will try to lure them closer for an ambush. The creature lures the curious and foolhardy into range by appearing briefly in the center of the lake, then disappearing under water, only to appear moments later within striking range.
Despite its undead state, the monster is extremely supple and can twist and curve its long body around, moving quickly through the water. It can coil its body underwater and strike like a snake up to 50 feet from the shore or up to 60 feet above the surface.
Such monsters are considered unique, yet there are some who speculate that there is more than one “serpent of the depths” in any world. Some claim to have sighted two separate sets of humps breaking the surface of a lake at once. Other sages maintain that the undead lake monster has an unnatural brood of little serpents, and that its attacks upon any who approach its lake are the equivalent of a mother protecting her young, but it is unclear how an undead creature could give birth to young.
The monster’s watery lair is said to be filled with the treasures of those it has pulled down to their deaths, but the lake which houses such a creature is always hundreds of feet deep, so it’s unlikely that any of that supposed treasure could ever be recovered. Even if items are somehow found, any armor or weapons in the hoard (unless magically protected) are likely to be rusted and useless.
Because it is undead, the lake monster has no natural life span. If killed, it will not provide any useful products. Its hide is tough enough to use for (leather) armor or a shield, but it has an oppressive stench that will force a character trying to use it to make hourly Constitution saves (DC 15) to avoid being poisoned.