Change Shape (Recharges after a Short or Long Rest). The xill can use a bonus action to magically polymorph into a giant phase spider, remaining in that form for up to 1 hour. It can revert to its true form as a bonus action. Its statistics, other than its size, are the same in each form. It can speak and cast spells while in giant phase spider form. Any equipment it is wearing or carrying in humanoid form melds into the giant phase spider form. It can’t activate, use, wield, or otherwise benefit from any of its equipment. It reverts to its humanoid form if it dies.

Magic Resistance. The xill has advantage on saving throws against spells and other magical effects.

Regeneration. The xill regains 20 hit points at the start of its turn if it has at least 1 hit point.

Spellcasting. The xill is a 17th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). It regains its expended spell slots when it finishes a short or long rest. It knows the following warlock spells:

Cantrips (at will): chill touch, eldritch blast, mage hand, poison spray

1st–5th level (4 5th-level slots): conjure animals (spiders only), crown of madness, dimension door, dispel magic, fear, enervation, giant insect, hold monster, insect plague, greater invisibility, vampiric touch, web, cloudkill

1/day each: power word kill, dominate monster, finger of death, eyebite

Ethereal Casting. While in the Ethereal Plane, the xill's spells can target and affect creatures on the Material Plane, or vice versa.

Ethereal Jaunt. As a bonus action, the xill can magically shift from the Material Plane to the Ethereal Plane, or vice versa.

Spider Climb. The xill can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Web Walker. The xill ignores movement restrictions caused by webbing.

Multiattack. The xill makes two claw attacks or two bite attacks. The first of these attacks that hits each round deals an extra 39 (6d12) poison damage to the target.

Claw (Humanoid Form Only). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 21 (4d8+3) piercing damage.

Bite (Giant Spider Form Only). Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 3) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 26 (4d12) poison damage and becoming poisoned on a failed save, or half as much damage on a successful one. If the poison 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.

Web (Giant Spider Form Only; Recharge 5-6). Ranged Weapon Attack: +8 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 15 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 10; hp 5; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).