The great catalogue desk of Khazan's flooded library under Lake Baratok: lacquered oak, brass handles, a hundred little drawers, and none of it a desk. It has eaten every reader but one, and filed their things.
AC
HP
Speed
CR
XP
14
157 (15d12+60)
20 ft
9
5000
STR
DEX
CON
INT
WIS
CHA
20 (+5)
8 (-1)
18 (+4)
10 (+0)
13 (+1)
8 (-1)
Saving throws: STR +9, CON +8
Skills: Stealth +7
Immune: acid
Condition immunities: prone
Senses: darkvision 60 ft; passive Perception 11
Traits
Adhesive. The Index adheres to anything that touches it. A Huge or smaller creature its pseudopods hit is Grappled.
Legendary Resistance (2/Day). When the Index fails a saving throw, it can choose to succeed instead, twice a day.
The Catalogue. Khazan's apprentices sat at the great desk to look things up. It looked them up too. Every drawer is full of a reader's things, filed.
Actions
Multiattack. The Index makes one Bite attack and two Pseudopod attacks.
Bite. Melee Attack Roll: +9, reach 5 ft. Hit: 16 (2d10 + 5) Piercing damage plus 9 (2d8) Acid damage.
Pseudopod. Melee Attack Roll: +9, reach 15 ft. Hit: 14 (2d8 + 5) Bludgeoning damage, and the target has the Grappled condition (escape DC 16).
Spray of Pages. Dexterity Saving Throw: DC 15, up to three creatures within 30 feet. Failure: 22 (5d8) Acid damage. Success: Half damage.
Lair actions
The Shelves Come Down. The library's shelves are rotten through. Two of them topple onto creatures the Index can sense: each makes a DC 15 Dexterity saving throw, taking 11 (2d10) Bludgeoning damage and falling Prone on a failure, or half the damage only on a success.
The Lake Comes In. The lake pushes through a crack in the wall onto one creature: it makes a DC 15 Strength saving throw, taking 13 (3d8) Cold damage and being pushed 10 feet on a failure, or half the damage only on a success.