Armor of Spiders (Mythic Trait; Recharges after a Short or Long Rest).

If Arasta is reduced to 0 hit points, she doesn't die or fall unconscious. Instead, she regains 200 hit points. In addition, Arasta's children immediately swarm over her body to protect her, granting her 100 temporary hit points.

Legendary Resistance (3/Day).

If Arasta fails a saving throw, she can choose to succeed instead.

Magic Resistance.

Arasta has advantage on saving throws against spells and other magical effects.

Spider Climb.

Arasta can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Web Walker.

Arasta ignores movement restrictions caused by webbing.

Actions

Multiattack.

Arasta makes three attacks: one with her bite and two with her claws.

Bite.

Melee Weapon Attack: +14 to hit, reach 5 ft., one creature. Hit: 20 (3d8 + 7) piercing damage, and the target must make a DC 21 Constitution saving throw, taking 32 (5d12) poison damage on a failed save, or half as much damage on a successful one. If the damage reduces the target to 0 hit points, the target is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.

Claws.

Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 17 (3d6 + 7) slashing damage.

Web of Hair (Recharge 4–6).

Arasta unleashes her hair in the form of webbing that fills a 30-foot cube next to her. The web is difficult terrain, its area is lightly obscured, and it lasts for 1 minute. Any creature that moves into the web or that starts its turn there must make a DC 21 Dexterity saving throw. On a failed save, the creature is restrained while in the web. A creature can use an action to make a DC 21 Strength check. On a success, it can free itself or a creature within 5 feet of it that is restrained by the web. This webbing is immune to all damage except magical fire. A 5-foot cube of the web is destroyed if it takes at least 20 fire damage from a spell or other magical source on a single turn.

Legendary Actions

Arasta 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. Arasta regains spent legendary actions at the start of its turn.

Claws. Arasta makes one attack with her claws.
Swarm (Costs 2 Actions). Arasta causes two swarms of spiders to appear in unoccupied spaces within 5 feet of her.
Toxic Web (Costs 3 Actions). Each creature restrained by Arasta's Web of Hair takes 18 (4d8) poison damage.

Mythic Actions

If Arasta's mythic trait is active, she can use the options below as legendary actions, as long as she has temporary hit points from her Armor of Spiders.

Swipe. Arasta makes two attacks with her claws.
Web of Hair (Costs 2 Actions). Arasta recharges Web of Hair and uses it.
Nyx Weave (Costs 2 Actions). Each creature restrained by Arasta's Web of Hair must succeed on a DC 21 Constitution saving throw, or the creature takes 26 (4d12) force damage and any spell of 6th level or lower on it ends.

Lair Actions

On initiative count 20 (losing initiative ties), Arasta can take a lair action to cause one of the following effects. She can't use the same effect two rounds in a row.

Arasta learns about any creature touching her webs. Each creature restrained by a web or Arasta's Web of Hair must make a DC 21 Intelligence saving throw. On a failed save, Arasta gains knowledge of a creature's name, race, where they consider home, and what brought them to her web.
Arasta casts the giant insect spell (spiders only). It lasts until she uses this lair action again or until she dies.

Regional Effects

The region containing Arasta's lair is warped by her presence, which creates one or more of the following effects:

Spiders and insects within 1 mile of Arasta's lair serve as her eyes and ears. Birds and other flying creatures are absent from the skies and occasionally found trapped in webs.
Within 1 mile of Arasta's lair, webs fill all 10-foot cubes of open space, so long as the webs can be anchored between two solid masses (such as walls or trees). The webs are flammable. Any webs exposed to fire burn away in 1 round. Any destroyed webs are magically repaired at the next dawn.

If Arasta dies, the spiders and insects lose their supernatural link to her. The webs remain, but they dissolve within 1d10 days.