Rejuvenation. If it dies, Yan'thelessia returns to life in 1d6 days and regains all its hit points. Only a wish spell can prevent this trait from functioning.
Legendary Resistance (3/day). If Yan'thelessia fails a saving throw, it can choose to succeed instead.
Spellcasting. Yan'thelessia is a 11th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 16, +7 to hit with spell attacks), and it needs only verbal components to cast its spells. It has the following wizard spells prepared:
Cantrips (at will): mage hand, minor illusion, Frostbite Ray of Frost
1st level (4 slots): charm person, detect magic, sleep
2nd level (3 slots): detect thoughts, hold person Phantasmal Force See Invisibility
3rd level (3 slots): Counterspell Fear
4th level (3 slots): blight, dimension door Vitriolic Sphere
5th level (2 slots): dominate person Dream Scrying
6th level (1 slot): Scatter Wall of Ice
Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one creature. Hit: 7 (1d6 + 5) piercing damage, and the target must make a DC 16 Constitution saving throw, taking 31 (7d8) poison damage on a failed save, or half as much damage on a successful one.
Poison Spray (Recharge 5-6). Yan'thelessia spits poison in a 20 ft cone. Each creature in that area must make a DC 14 Constitution saving throw, taking 21 (6d6) poison damage on a failed save, or half as much damage on a successful one
Yan'thelessia can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Yan'thelessia regains spent legendary actions at the start of its turn.
Attack. Yan'thelessia makes a bite attack or casts a cantrip
Sacrifice. Yan'thelessia can absorb the lifeforce of a willing creature, dealing 20 psychic damage to it and restoring 20 hit points
Inflict Fear (Costs 2 Actions). Each Creature in a 30ft cone must succeed on a DC 16 Wisdom saving throw or become frightend for 1 Minute. A creature frightend this way can repeat the saving throw at the end of each of its turns.
Lair and Lair Actions
When fighting inside its lair, Yan'thelessia can invoke the ambient magic to take lair actions. On initiative count 20 (losing initiative ties), Yan'thelessia can take one lair action to cause one of the following effects:
Hypnotic Pattern. The lights in the lair begin flashing and changing colors. Each creature in the area who sees the pattern must make a DC 12 Wisdom saving throw. On a failed save, the creature becomes charmed for the duration. While charmed by this spell, the creature is incapacitated and has a speed of 0.
The spell ends for an affected creature if it takes any damage or if someone else uses an action to shake the creature out of its stupor.
Mind Spike. Sharp maddening sounds of hundreds of voices fill the cave. Each creature in the area who can hear the voices must make a DC12 Wisdom saving throw. Taking 3d8 psychic damage on a failed save. Yan'thelessia always knows the creatures location as long as it is on the same plane. While Yan'thellesia has this knowledge, the target can’t become hidden from her and if it’s invisible, it gains no benefit from that condition against her. The effect lasts for 24 hours