On the northern border of the Old Kingdom, Lake Arethal stands as silent witness to the passing of orc bands and barbarian tribes. Despite legends of great wisdom to be gained there, no one has returned from its shores in over three centuries. You see, Lake Arethal is guarded by a pair of immortal sentinels, serpents of the water and the moon — lovers named Scara and Nelox.
While Scara is at best indifferent toward those who would seek answers to long-forgotten questions, her mate Nelox takes pleasure in tormenting aspirants with riddles. Any mortal who fails to answer is scoured with moonlight and consumed. Scara does her best to warn travelers away from the water, but if they ignore her and meet Nelox, she is willing to leave them to their fate.
When fighting inside its lair, a spirit serpent can invoke the ambient magic to take lair actions. On initiative count 20 (losing initiative ties), the spirit serpent takes a lair action to cause one of the following effects (Scara and Nelox get one lair action between them):
The water level of the lake raises or lowers by 5 feet. Any non-aquatic, non-flying creature in the water or within 20 feet must make a DC 16 Strength saving throw or become prone and restrained for 1 round. The spirit serpent can’t use this lair action again until it uses another one.
The spirit serpent reaches into the mind of a single creature that is touching the water of the lake. That creature must make a DC 16 Wisdom saving throw. On a failure, the serpent telepathically asks a question and the creature must answer truthfully.
The spirit serpent targets a single creature with a bolt of moonlight. The target must make a DC 16 Dexterity saving throw. On a failure, it takes 11 (2d10) radiant damage, or half as much on a success. The spirit serpent can’t use this lair action again until it uses another one.
Huge monstrosity, neutral (Scara) or neutral evil (Nelox) Armor Class 16 (natural armor) Hit Points 133 (14d12+42) Speed 30 ft., swim 60 ft., fly 20 ft. (hover)
Rejuvenation. If it dies, the spirit serpent returns to life in 1d6 days and regains all its hit points. Only a wish spell can prevent this trait from functioning.
Innate Spellcasting. The spirit serpent’s innate spellcasting ability is Wisdom (spell save DC 16). The spirit serpent can innately cast the following spells, requiring no material components: At-will: create or destroy water, thaumaturgy. 3/day: moonbeam. 1/day: *call lightning.
Multiattack. When the spirit serpent takes the Attack action, it can make a bite and a tail swipe.
Bite. Melee attack: +7 to hit, reach 5 ft., one target. Hit: 12 (2d8+3) piercing and 18 (4d8) radiant damage.
Tail Swipe. Melee attack: +7 to hit, reach 10 ft., one target. Hit: 10 (1d12+3) slashing and 5 (1d8) radiant damage. The target must make a DC 17 Strength saving throw or become prone.
The spirit serpent can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The spirit serpent regains spent legendary actions at the start of its turn.
Bite. The spirit serpent makes a bite attack.
Move. The spirit serpent moves its speed.
Tail Whirl (2 actions). Every creature within 10 feet must make a DC 18 Dexterity saving throw. On a failure, they take 10 (1d12+3) slashing damage and become prone. On a success, they take half as much damage and do not become prone.