Aracnissith

Ancient Presence.
The Ancient Arachnid Serpent has advantage on saving throws against being charmed or frightened.

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

Web Walker.
The Ancient Arachnid Serpent ignores movement restrictions caused by webbing.

Anti-Magic Carapace.
The Ancient Arachnid Serpent is surrounded by an aura of anti-magic. Within a 30-foot radius of the creature, spells cannot be cast, summoned creatures disappear, and magical items become mundane. The aura moves with the Ancient Arachnid Serpent. If the Ancient Arachnid Serpent is in an area of anti-magic created by a spell or another effect, the Ancient Arachnid Serpent’s aura suppresses that effect.

Multiattack.
The Ancient Arachnid Serpent makes three attacks: one with its bite and two with its legs.

Bite.
Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 25 (3d10 + 8) piercing damage plus 14 (4d6) poison damage. The target must succeed on a DC 21 Constitution saving throw or be poisoned for 1 minute. While poisoned in this way, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Legs.
Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 8) slashing damage.

Web (6).
Ranged Weapon Attack: +10 to hit, range 30/60 ft., one target. Hit: The target is restrained by webbing. As an action, the restrained target can make a DC 18 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 15; hp 20; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).

Legendary Actions

The Ancient Arachnid Serpent 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 Ancient Arachnid Serpent regains spent legendary actions at the start of its turn.

Detect.
The Ancient Arachnid Serpent makes a Wisdom (Perception) check.

Tail Swipe.
The Ancient Arachnid Serpent makes a tail attack: Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 22 (3d10 + 8) bludgeoning damage.

Web Spray (Costs 2 Actions).
The Ancient Arachnid Serpent recharges and uses its Web attack.