Ambitious Assassin

Small or Medium Humanoid, Neutral Evil

AC: 16

Initiative: +4 (14)

HP: 91 (14d8 + 28)

Speed: 40 ft., climb 40 ft.

STR: 10 (+0)
DEX: 18 (+4)
CON: 15 (+2)
INT: 18 (+4)
WIS: 14 (+2)
CHA: 16 (+3)

Saving Throws: Dex +7, Int +7

Skills: Deception +9, Perception +5, Stealth +10

Condition Immunities: charmed; frightened

Senses: blindsight 10 ft., Passive Perception 15

Languages: Common, thieves' cant, plus any one language

CR: 5 (XP 1,800; PB +3)

Legendary Resistance (2/Day).

If the assassin fails a saving throw, it can choose to succeed instead.

Actions

Multiattack.

The assassin makes two Poison Blade attacks.

Poison Blade.

Melee or Ranged Weapon Attack: +7 to hit, reach 5 ft. or range 60 ft., one target. Hit: 6 (1d4 + 4) piercing damage plus 5 (1d10) poison damage.

Reactions

Uncanny Dodge.

When an attacker the assassin can see hits the assassin with an attack, the assassin halves the attack's damage against it.

Legendary Actions

Legendary Action Uses: 3. Immediately after another creature's turn, this creature can expend a use to take one of the following actions; it regains all expended uses at the start of each of its turns.

Cunning.

The assassin escapes nonmagical restraints and ends the grappled condition on itself, then moves up to its speed without provoking opportunity attacks.

Stab (Costs 2 Actions).

The assassin makes one Poison Blade attack.

Vanishing Escape (Costs 3 Actions).

The assassin creates a sudden distraction, such as a cloud of disorienting smoke or flash of dazzling light, filling a 10-foot cube within 5 feet of the assassin. Each creature of the assassin's choice in that area takes 9 (2d8) psychic damage, and the assassin has the invisible condition. This invisibility lasts until the end of the assassin's next turn.

Lair Actions

On initiative count 20 (losing initiative ties) while in its lair, the villain can take one of the following lair actions; the villain can't take the same lair action two rounds in a row:

Distraction. A shrill sound fills the lair. Creatures that can hear it must succeed on a DC 15 Constitution saving throw or lose concentration.
Snares. Hidden snares target up to three creatures the villain can see in the lair. Each creature must succeed on a DC 15 Strength saving throw or have the grappled condition (escape DC 15) and the restrained condition until it escapes or the snare holding it is destroyed. Each snare has AC 11, 10 hit points, and immunity to poison and psychic damage.
Traps. Traps target up to two creatures the villain can see in the lair. Each target must succeed on a DC 15 Dexterity saving throw or be struck by a trap. The villain rolls a number of d6s equal to its proficiency bonus, and struck targets take damage equal to the total. The damage is bludgeoning, piercing, slashing, fire, or poison, chosen by the villain when the villain uses this action.

Regional Effects

The region containing a villain's lair is warped by their presence, creating one or more of the following effects:

Corruption Abounds. While within 3 miles of the lair, creatures that aren't members of the villain's organization have disadvantage on ability checks that rely on the Insight, Perception, and Persuasion skills.
Potent Toxins. When members of the villain's organization deal poison damage while within 3 miles of the lair, they ignore the target's resistance to poison damage.

If the villain dies, these effects fade over the course of 1d10 days.

Find this creature in the encounter builder

Source: The Book of Many Things, p. 45 - Licenses and attribution