Aldius, the Bookwyrm

Arcane Precognition. After Aldius rolls initiative, roll a second d20. Rather than adding Dexterity, add Aldius’ spellcasting modifier (INT) to this roll. This is their Arcane Initiative. This has no mechanical function on its own, but other abilities may refer to using Aldius’ Arcane Initiative such as the Conjure Living Spell ability.

Immutable Form. Aldius is immune to any spell or effect that would alter its form.

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

Shelving Body.  Aldius cannot become small while they have greater than half their hit points, and they cannot become huge while they have fewer than half their hit points. Aldius' Strength is determined by their size.

• 12 if Tiny
• 14 if Small
• 16 if Medium
• 18 if Large
• 20 if Huge

While Aldius is small or tiny, it can Hide as a bonus action and has advantage of Dexterity saving throws. While Aldius is large or huge, it has advantage on Constitution saving throws.

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

Multiattack. Alidus makes three attacks: one with a cantrip and two with its claws.

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: (2d6 + X) slashing damage.  

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft. (reach 15ft, if huge), one target. Hit: (5d6 + X) slashing damage. X is Aldius' current Strength modifier.

Spellcasting.

Aldius is an is a 11th-level Construct spellcaster. Its spellcasting ability is Intelligence (spell save DC 16, +8 to hit with spell attacks). Aldius has the following spells prepared:

• Cantrips (at will): Acid Splash, Mage Hand, Message, Mending, Sword Burst

• 1st level (4 slots): Catapult, Chromatic Orb, Detect Magic (C)

• 2nd level (3 slots): Magic Mouth, Silence (C), Tasha's Mind Whip

• 3rd level (3 slots): Dispel Magic, Hypnotic Pattern (C), Lightning Bolt

• 4th level (3 slots): Confusion, Gravity Sinkhole, Vitriolic Sphere

• 5th level (2 slots): Animate Objects (C), Cloudkill (C),  Bigby's Hand (C)

• 6th level (1 slot): Blade Barrier

Origami Defense. As a reaction when hit by an attack or at the beginning of it’s turn, Aldius can change their size up to once in one direction. If Aldius' size decreases this way, that attack gets -d8 to the roll.

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

Cantrip. Aldius casts a cantrip. If Aldius chooses the Mending cantrip, they restore 2d6 hit points.

Hide & Hunt. Aldius may change sizes. If they do, they may move up to their speed without taking attacks of opportunity. If they are medium or smaller, Aldius may then make a Dexterity (Stealth) check.

Tail Attack. Aldius makes a tail attack.

Cloud of Pages (Costs 2 Actions). Aldius beats their wings and disperses into a cloud of pages; Aldius size drops by one. Each creature within 20 feet of Aldius must succeed on a DC 16 Dexterity saving throw or take (2d6 + 4) slashing damage. Aldius decreases in size. Until Aldius' next turn, this area is rough terrain and is heavily obscured.