Tesso, the Rat Overlord 鉄鼠

Large fiend (demon, yokai), chaotic evil

Armor Class 19 (natural armor)

Hit Points 257 (23d10 + 115)

Speed 40 ft., climb 30 ft.

STR 22 (+6) · DEX 18 (+4) · CON 20 (+5) · INT 14 (+2) · WIS 16 (+3) · CHA 20 (+5)

Saving Throws Str +12, Con +11, Cha +11

Skills Athletics +12, Intimidation +11, Perception +9, Stealth +10

Damage Resistances cold, lightning; bludgeoning, piercing, and slashing from nonmagical attacks

Damage Immunities fire, poison

Condition Immunities poisoned

Senses darkvision 120 ft., passive Perception 19

Languages Yokai Speech, Abyssal, Common; telepathic link with rats

Challenge 15

Traits

Iron Hide (Enhanced). Tesso has resistance to bludgeoning, piercing, and slashing from all sources unless the attack is made with a magic or adamantine weapon (you can adjust if you want standard fiendish resistances).

Rat Mastery (Greater). Any rat swarm, giant rat, or ratlike fiend within 100 feet automatically obeys Tesso. As a bonus action, Tesso can command these rats (move, guard, attack, etc.), which act on his turn.

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

Actions

Multiattack. Tesso makes three attacks: two with Claw and one with Bite, or two with Claw and uses Gnawing Hex if available.

Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target.

Hit: 15 (2d8 + 6) slashing damage. If the target is a creature, it must succeed on a DC 18 Constitution saving throw or be cursed until the end of its next turn. While cursed, the target’s speed is reduced by 10 feet and it cannot take the Dash action.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target.

Hit: 17 (2d10 + 6) piercing damage plus 10 (3d6) necrotic damage. The target must succeed on a DC 19 Constitution saving throw or become diseased. While diseased, the target has disadvantage on Strength and Dexterity saving throws. The target can repeat the save at the end of each long rest, ending the disease on a success.

Summon Rat Swarm (Greater) (Recharge 5-6). Tesso calls forth a wave of rats. One swarm of rats (MM p. 339) or a swarm of cranium rats (if you prefer an underdark flair) appears in an unoccupied space within 30 feet, acting on Tesso’s initiative immediately after him, under his telepathic control. The swarm remains for 1 hour, until Tesso dismisses it, or until Tesso drops to 0 hit points.

Devour Manuscript (Enhanced) (1/Day). Tesso targets a nonmagical or low-tier magical written text (book, scroll) within 5 feet, devouring it with supernatural speed. Any knowledge or secrets are lost forever unless recovered by powerful magic like wish (PHB, p. 288).

Gnawing Hex (Recharge 5-6). Tesso points at one creature within 60 feet that can see or hear him. The target makes a DC 17 Constitution saving throw. On a failed save, it takes 21 (6d6) necrotic damage and is stunned until the end of Tesso’s next turn as phantom rats bite at its mind and body. On a success, the target takes half damage and isn’t stunned.

Bonus Actions

Tail Swipe. Tesso makes a quick tail lash at a creature within 5 ft.: Melee Weapon Attack: +12 to hit, one target.

Hit: 10 (1d8 + 6) bludgeoning damage, and the target must succeed on a DC 18 Strength saving throw or be pushed 5 feet away.

Reactions

Vengeful Lash (Greater). When a creature within 5 feet misses Tesso with a melee attack, Tesso can make one Claw attack against that creature with advantage.

Legendary Actions

If you want Tesso to be even more formidable, grant him 1 Legendary Action at the end of another creature’s turn:

Swarm Surge (Costs 1 Action). Tesso commands any rat swarms he’s summoned to move up to half their speed and immediately make one Swarm attack if able.

Lore

Tesso’s decades (or centuries) of demonic wrath have allowed him to grow mightier, commanding colossal rat hordes and inflicting potent curses.

Tesso 鉄鼠 The legend of Tesso (literally “Iron Rat”) connects to a real historical figure: the Buddhist monk Raigo Ajari. His story is documented in Buddhist chronicles, which tell of Emperor Shirakawa, fervently longing for an heir. Hoping for divine favor, the emperor requested the head priest of Mii-dera to have his monk, Raigo, pray day and night for the birth of a son. Renowned for his piety and spiritual power, Raigo took it upon himself, with utmost devotion, to help the emperor. When Emperor Shirakawa asked Raigo what reward he desired if the prayers were successful, the monk asked that Mii-dera be allowed to take on fifty new novices, thereby expanding the temple.

After many nights of prayer, Fujiwara no Kenshi conceived, and Prince Taruhito was born. However, when it came time to grant the promised reward, complications arose. Rival monks from Enryaku-ji pressured the emperor to renege - their motives uncertain, though likely driven in jealousy and unwillingness to let Mii-dera become the dominant temple of Shiga Prefecture. Feeling betrayed and enraged, Raigo declared a hunger strike to protest the ruler’s injustice. In the end, he died, cursing the emperor and his bloodline. Rumor has it that, in his final days, Raigo lost his grip on sanity, casting dark curses on all his adversaries in the neighboring temple and praying to demons.

From Yokai Bestiary 5E, p. 12 · Fern Flower Press