WARNING: Mature Content

I obviously love Eureeka’s Castle. Let’s be plain about that. I really do—it was a formative thing of my youth, I still remember the theme song, and I think it is a wonderful children’s television program. If you have children, or if you are yourself a child, consider queuing up an episode or two instead of reading what’s below. If you are like me however (a jaded adult with a treasure chest of complexes) then by all means continue onward.

The mice of Eureeka’s Castle went hungry many times during the rule of their second master, turning to cannibalism and eating their weakest kin in order to survive. Things did not improve greatly when their friend mage rose to power—she quickly discerned what the rodents had done to one another without her presence, using the darkness in their hearts as a gateway to flood their bodies with vile energies. Thus Eureeka reworked the mice of the castle into her puppets, subduing their will entirely. The corrupted little monstrosities have become her trusted spies, scurrying all throughout the music box and returning to her with every secret and whisper they can find.

Design Notes: Sneaky little evil magic spy mice! I decided to make these especially difficult to kill because we’re looking at adventurers between 7th and 13th level rolling around in Eureeka’s Castle D&D 5E so they’ve got Cunning Action, Evasion, and Regeneration. Then to make people want to kill them I rolled in some annoying cantrips, a smattering of actually problematic spells (which most folks will easily beat the spell save DC on, so it’s definitely not going to be relevant unless several castle mice are around trying). Let’s take a look at the numbers! The DMG comes in at 4.5 and the Blog of Holding at 4.875: that means a solid CR 4!

Castle Mouse

Tiny monstrosity, neutral evil
Armor Class 13
Hit Points 36 (8d4+16)
Speed 20 ft.

Saving Throws Dex +5, Con +4
Skills Insight +4, Perception +6, Stealth +7
Damage Resistances necrotic, poison
Condition Immunities frightened
Senses darkvision 30 ft., passive Perception 16
Languages Common
Challenge 4 (1,100 XP)

Cunning Action (1/Turn). The castle mouse can use a bonus action to take the Dash, Disengage, or Hide action.

Evasion. If the castle mouse is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the castle mouse instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Innate Spellcasting. The castle mouse’s innate spellcasting ability is Wisdom (spell save DC 12, +4 to hit with spell attacks). The castle mouse can innately cast the following spells, requiring no material components:

• At will: chill touch, eldritch blast
• 3/day each: detect thoughts, mage hand, minor illusion
• 1/day each: charm person, clairvoyance, detect magic, invisibility, suggestion

Keen Smell. The castle mouse has advantage on Wisdom (Perception) checks that rely on smell.

Regeneration. The castle mouse regains 10 hit points at the start of its turn if it has at least 1 hit point.

ACTIONS
Bite. Melee Weapon Attack: +0 to hit, reach 5 ft., one target. Hit: 1 piercing damage plus 5 (2d4) necrotic damage.