Actions

Multiattack. Harissa, Yochlol Princess makes three melee attacks.

Death Lance. Melee Weapon Attack: 1d20+10 to hit, reach 5 ft., one target. Hit: 8 1d6+5 piercing damage plus 18 4d8 necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage it takes. This reduction lasts until the target finishes a long rest. The target dies if its hit point maximum is reduced to 0.

Slam (Bite in Spider Form). Melee Weapon Attack: 1dd20+10 to hit, reach 5 ft. (10 feet in demon form), one target. Hit: 5 1d6+5 bludgeoning (piercing in spider form) damage plus 21 6d6 poison damage.

Mist Form. Harissa, Yochlol Princess transforms into toxic mist or reverts to its true form. Any equipment it is wearing or carrying is also transformed. It reverts to its true form if it dies.
While in mist form, Harissa, Yochlol Princess is incapacitated and can't speak. It has a flying speed of 30 feet, can hover, and can pass through any space that isn't airtight. It has advantage on Strength, Dexterity, and Constitution saving throws, and it is immune to nonmagical damage.

While in mist form, Harissa, Yochlol Princess can enter a creature's space and stop there. Each time that creature starts its turn with Harissa, Yochlol Princess in its space, the creature must succeed on a DC 14 Constitution saving throw or be poisoned until the start of its next turn. While poisoned in this way, the target is incapacitated.   Summon Demon (3/Day)
Harissa, Yochlol Princess chooses what to summon and attempts a magical summoning.
Harissa, Yochlol Princess has a 50 percent chance of summoning one yochlol.
A summoned demon appears in an unoccupied space within 60 feet of its summoner, acts as an ally of its summoner, and can't summon other demons. It remains for 1 minute, until it or its summoner dies, or until its summoner dismisses it as an action.

Reactions

Parry. Harissa, Yochlol Princess adds 3 to its AC against one melee attack that would hit her. To do so, Harissa, Yochlol Princess must see the attacker and be wielding a melee weapon.

Legendary Actions

Harissa, Yochlol Princess 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. Harissa, Yochlol Princess regains spent legendary actions at the start of its turn.

Death Lance. Harissa, Yochlol Princess makes one attack with her death lance.
Compel Demon (Costs 2 Actions). An allied demon within 30 feet of Harissa, Yochlol Princess uses its reaction to make one attack against a target of Harissa, Yochlol Princess's choice that she can see.
Cast a Spell (Costs 1–3 Actions). Harissa, Yochlol Princess expends a spell slot to cast a 1st-, 2nd-, or 3rd-level spell that she has prepared. Doing so costs 1 legendary action per level of the spell.