Sources
CR
Type
Size
Alignment
Movement

Decaton Modron

Large Construct, Lawful Neutral

Armor Class 16 (natural armor)
Hit Points 144 (17d10 + 51)
Speed 30 ft., fly 30 ft. (hover)
STR
18
+4
DEX
12
+1
CON
16
+3
INT
15
+2
WIS
15
+2
CHA
11
+0
Saving Throws INT +5, WIS +5
Skills Perception +8
Condition Immunities Charmed, Frightened
Senses truesight 120 ft., passive Perception 18
Languages Modron, telepathy 120 ft.
Challenge 8 (3,900 XP)

Traits

Axiomatic Mind. The decaton can't be compelled to act in a manner contrary to its nature or its instructions.

Combat Ready. The decaton has advantage on initiative rolls.

Disintegration. If the decaton dies, its body disintegrates into dust, leaving behind anything it was carrying.

Spellcasting

Spellcasting. The decaton casts one of the following spells, requiring no material components and using Intelligence as the spellcasting ability (spell save DC 13):

At will: detect magic, dispel magic, mending (as an action)

1/day each: plane shift (self only), protection from evil and good

Actions

Multiattack. The decaton makes three Tentacle attacks.

Tentacle. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit:15 (2d10 + 4) piercing damage, and if the target is a Medium or smaller creature, it has the grappled condition (escape DC 14). Until this grapple ends, the decaton can't use this tentacle against other targets. The decaton has ten tentacles, each of which can grapple one target.

Lightning Rays (Recharge). The decaton unleashes a barrage of lightning bolts from its eyes. Each creature within 30 feet of the decaton must make a DC 13 Dexterity saving throw, taking 38 (7d10) lightning damage on a failed save, or half as much damage on a successful save.

Lore

Constructed on the plane of Mechanus, modrons are partially mechanical beings that belong to a strict hierarchy. Each modron dutifully obeys commands from the rank directly above it and in turn acts as the superior to the rank directly below it, passing down commands from paragons of law to the lowliest monodrone. While most modrons are the lower-ranked base modrons—monodrones, duodrones, tridrones, quadrones, and pentadrones—the upper-tier hierarch modrons hold leadership positions,…