“Ah, Ambrose’s lost play. A first edition! The play is utter trash, of course, but an important historical find. What do you want in exchange—besides your lives, of course?”

Once the preeminent scholar in the world’s most glorious library, Chilblain’s soul was crushed when the library was burned. With nothing to lose, Chilblain followed the path of the Undying, becoming a lich. She came to the Wasteland in search of the thousands of books that were lost in the fire. Over the centuries, she has reconstructed a Wasteland replica of her beloved library, although many of its original titles still elude her grasp.

Finding Chilblain: Chilblain resides in the Crawling Library, a perfect replica of her lost library—except that it walks on hundreds of metal legs. It marches, slowly and eternally, over the mountains of the Shambles, keeping just ahead of the Outer Rim and the Gyre behind it. For those whom Chilblain welcomes inside, the Crawling Library counts as a haven.

Dealing with Chilblain: The Crawling Library is one of the most complete archives in the multiverse, with many books that can’t be found anywhere else. Researchers may be able to find unique tomes and spellbooks (many of which contain rare spells). Despite its excellence as a library, Chilblain is still eager for replacements for the hundreds of volumes lost when her library burned. She’d offer unique rare spells and tomes—perhaps even her copy of the Book of the Undying—in exchange for a rich enough trove. Woe betide those who come into her library without something valuable to trade; they rarely leave alive.

Chilblain Challenge 10

Medium undead

AC 12 (15 with mage armor)

HP 119 (14d8 + 56; bloodied 59)

Speed 30 ft.

Initiative Dex +3 (13), Insight +3 (13), Perception +7 (17)

STR DEX CON INT WIS CHA

12 (+1) 16 (+3) 18 (+4) 20 (+5) 16 (+3) 16 (+3)

Proficiency +4; Maneuver DC 15

Saving Throws Str +1, Dex +3, Con +4, Int +9, Wis +7, Cha +3

Skills Arcana +9, History +9, Perception +7, Religion +9

Damage Immunities necrotic, poison

Condition Immunities fatigue, paralyzed, poisoned

Senses passive Perception 17

Items Book of the Undying

Languages Common, Deep Speech, Draconic, Ignan, Infernal

Protective Rune. Chilblain’s vestments are warded with a protective rune. When she fails a saving throw, she can use a reaction to expend the rune, succeeding on the saving throw instead.

Contingency. When Chilblain takes damage, her soulfire shield spell activates (no action required). While the spell is active, Chilblain has resistance to radiant damage and a creature within 5 feet of her takes 2d8 necrotic damage when it hits her with a melee attack.

Spellcasting. Chilblain is a 14th-level spellcaster. Her spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). She has the following wizard spells prepared:

Cantrips (at will): lich bolt, mage hand, prestidigitation

1st level (4 slots):Chilblain’s frost shield, mage armor

2nd level (3 slots):Chilblain’s blindness and deafness of the grave, Chilblain’s freezing hold person

3rd level (3 slots):Chilblain’s whiteflame fireball, Chilblain’s tiny study nook

4th level (3 slots):Chilblain’s trans-dimension door, Chilblain’s soulfire shield

5th level (3 slots):Chilblain’s hungry cloudkill, Chilblain’s truename scrying

6th level (1 slots):Chilblain’s magnetic chain lightning, Chilblain’s chained contingency

7th level (1 slots):Chilblain’s rousing sequester

Rejuvenation. If she has a soul vessel, when Chilblain is destroyed she gains a new body in 10 days, regaining all of her hit points. Her new body forms within 10 feet of the soul vessel.

Undead Nature. Chilblain doesn’t need air, sustenance, or sleep.

ACTIONS 

Lich Bolt (Cantrip; V, S).Up to three creatures within 60 feet must succeed on DC 17 Dexterity saving throws or take 10 (3d6) lightning damage. The damage increases by 1d6 if there are 2 targets and 2d6 if there is one target.

Chilblain’s whiteflame fireball (3rd-Level; V, S, M).Frost streaks from Chilblain to a point within 120 feet and explodes in a 20-foot radius, spreading around corners. Each creature in the area makes a DC 17 Dexterity saving throw, taking 21 (6d6) cold damage on a failed save or half damage on a success. 

Chilblain’s magnetic chain lightning (6th-Level; V, S, M).A creature within 120 feet, along with up to 3 additional creatures within 30 feet of it, must each make a DC 17 Dexterity saving throw, taking 45 (10d8) lightning damage on a failed save or half damage on a success. Creatures holding metal objects or wearing metal armor make their saving throw against this spell with disadvantage. Each creature that fails its saving throw against the spell is either pushed up to 90 feet toward or 60 feet directly away from Chilblain.

Read more at this site