Changeling Cult Leader

Legendary Resistance (3/Day). If the cult leader fails a saving throw, it can choose to succeed instead. 

Shapechanger. The cult leader can use its action to polymorph into a Small or Medium humanoid it has seen, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Ambusher. The cult leader has advantage on attack rolls against any creature it has surprised.

Surprise Attack. If the cult leader surprises a creature and hits it with an attack during the first round of combat, the target takes an extra 10 (3d6) damage from the attack.

Spellcasting. The cult leader is a 15th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It has the following paladin spells prepared:

At Will: invisibility

1st level (4 slots): command, compelled duel, searing smite (necrotic), cure wounds,

2nd level (3 slots): hold person, magic weapon

3rd level (3 slots): dispel magic, aura of vitality, spirit shroud

4th level (3 slots): banishment, staggering smite

5th level (1 slots): banishing smite

Multiattack. The cult leader makes two attacks with the Ceremonial Daggers or one with the Bloodied Katana.

Bloodied Katana. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (2d6 + 4) slashing damage. On hit, creature must make a DC 13 Con save or become stunned until the end of its next turn.

Ceremonial Daggers. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 7 (1d10 + 10) slashing damage. Plus [rollable](2d4) Poison Damage.

Read Thoughts. The Cult Leader magically reads the surface thoughts of one creature within 60 feet of it. The effect can penetrate barriers, but 3 feet of wood or dirt, 2 feet of stone, 2 inches of metal, or a thin sheet of lead blocks it. While the target is in range, the doppelganger can continue reading its thoughts, as long as the doppelganger's concentration isn't broken (as if concentrating on a spell). While reading the target's mind, the doppelganger has advantage on Wisdom (Insight) and Charisma (Deception, Intimidation, and Persuasion) checks against the target.

The cult leader 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 cult leader regains spent legendary actions at the start of its turn.

Guise of Darkness. The cult leader casts the Invisibility spell on himself.

Step through Dimensions. The cult leader can move double its speed without taking attacks of opportunity. 

Words of Despair (Costs 2 Actions). The cult leader begins to chant in Abyssal and all creatures within a 30ft radius of him must make a DC 18 charisma save or be stunned until the end of his next turn.