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Rethiarius the Sentinel

Huge construct (robot, legendary), neutral evil
Armor Class 17 (natural armour)
Hit Points 247 ( 15d12+90 )
Speed Hover: 80ft

28 +9
10 0
22 +6
14 +2
10 0
14 +2

Saving Throws Dex +4, Con +8, Wis +5, Cha +7
Skills Perception +9
Damage Vulnerabilities lightning
Damage Immunities necrotic, poison, psychic; bludgeoning, piercing, and slashing from nonmagical attacks that aren't skymetal (adamantine)
Condition Immunities charmed, deafened, exhaustion, frightened, paralyzed, petrified, poisoned
Senses blindsight 30 ft., passive Perception 19
Languages Gothic and dozens of others, sometimes including Common
Challenge 15 (13,000 XP)

  • Voltaic Capacitor. Rethiarius heals from lightning damage. If already at full hps, any excess becomes a non-regenerating force field of temporary hit points.
  • Focused. Rethiarius will focus its non-aoe attacks on one target until that target stops moving. This single mindedness can be used to keep the shield focused away from the rest of the party.
  • Impenetrable Shield. Rethiarius projects a shield directly in front of it; while active, it is immune to all damage from this angle. At the start of each of its turns, Rethiarius decides which way the shield faces and whether the shield is active. An EMP device or lightning attack brings the shield down until rethiarius has a long rest.
  • EMP Shield. Rethiarius has advantage on saving throws against attacks or effects that seek to shut it down. Even if it fails, the effect only lasts 1 round before the robot reboots. If attacked while in a shutdown state it instantly recovers and defends itself.


  • Rubiks Reconfiguration. At the start of combat, roll a 1d24 to determine which attacks trigger this round. Rethiarius always uses Expand. None of Rethiarius's attacks provoke attacks of opportunity. If there are no targets in range of Expand, it instead uses a half-strength Chain-Lazer until a target is within range of Expand.
  • Expand (Cube). All creatures within 15 feet of Rethiarius must make a DEX saving throw or take 3d8+9 ) piercing damage.
  • Roll (Sphere, Natural 15+). All creatures within 20 feet of Rethiarius must make a DEX saving throw or take 3d8+9 ) bludgeoning damage and be knocked prone.
  • Chain-Lazer (Triangle, Natural 18+). Rethiarius creates a perfectly straight line of blue energy that bounces toward a target it can see within range and one more target and itself forming a perfect triangle; each target must be within 30 feet of the first target. A target can be a creature or an object and can be targeted by only one of the lines. A target must make a DEX saving throw. The target takes 3d8 lightning damage on a failed save, or half as much damage on a successful one. Rethiarius heals for as much lightning damage as it dealt in total. 
  • Mutation Beam (Cube, Natural 20+ and Timer 1d4). Rethiarius attacks up to three separate creatures within 120 feet (that it is not directly facing). Each creature targeted must make a DEX saving throw, taking 22 ( 3d8+9 ) poison damage (as radiation) on a failed save, or half as much damage on a successful one. Living creatures must also succeed on a CON saving throw or gain a harmful mutation that fully develops over the course of 1d4 rounds (or when the timer ends). A target can gain only one harmful mutation per encounter with Rethiarius. The mutation troubles the target for one day and then fades.


Disassemble. The first time Rethiarius is reduced to half its starting hit points it explodes into its component parts of 27 smaller cubes and begins regenerating. All creatures within 30 feet must make a DEX saving throw or take 1d8 bludgeoning damage, or none on a successful save. Each cube has AC 18 and 10 hit points. On Rethiarius' next initiative count it reforms and heals 5 hit points for each cube that was not destroyed.


Legendary Actions

Rethiarius can take 3 legendary actions, choosing from any of its available actions (roll 1d20 to see which are available at any given time), as well as its lair action option or it can used its movement. Only one legendary action option can be used at a time and only at the end of another creature's turn. Rethiarius regains spent legendary actions at the start of its turn.

Lair Actions

On initiative count 20 (losing initiative ties), Rethiarius takes a lair action to cause one of the following effects (or rolls randomly 1d3 , rerolling sequential effects Rethiarius can’t use the same effect two rounds in a row:

  • Interwebs. Rethiarius glimpses the future, so it imposes a bane (-1d6) on all saving throws against its attacks until initiative count 20 on the next round.
  • Electric Blue. Rethiarius electrifies an area of the floor. The area is determined randomly (either a 10-foot square or a zone if using zone combat). Each creature in that area takes 5 lightning damage.
  • The Matrix. One creature Rethiarius can see within 120 feet of it must succeed on a CHA saving throw or be banished to a digital plane, a different plane of existence Rethiarius has programmed into being. To escape, the creature must use its action to make a CHA check. If the creature wins, it escapes the digital plane. Otherwise, the effect ends on initiative count 20 on the next round. When the effect ends, the creature reappears in the space it left or in the nearest unoccupied space if that one is occupied.

Rethiarius will be found in technological ruins of the Ancients, usually near a significant discovery like an auto-forge, interplanetary gate etc.

The Rethiarius appears as a glowing red cube made up of smaller cubes (imagine a Rubik's cube the size of pickup truck). It was originally programmed to direct the various Zhev in their duties, however, it has been corrupted and reprogrammed by a Telepath (Typhon Psychocratis).

Tactical Combat Encounter (Zones)

Details on this encounter can be found here.


01–10Deformed leg: Your speed on foot is reduced by 5 feet. You must succeed on a DC 10 Dexterity saving throw after using the Dash action. If you fail the save, you fall prone.
11–20Deformed face/appearance: You are deformed or disfigured to the extent that it can’t be easily concealed. You have disadvantage on Charisma (Persuasion) checks and advantage on Charisma (Intimidation) checks.
21–30Deformed arm/hand: You have disadvantage with any roll requiring you to use that arm or hand (including wielding two-handed weapons).
31–40Malformed brain: You have disadvantage on all Intelligence checks.
41–45Mentally vulnerable: You have disadvantage on Intelligence saving throws.
46–50Slow and lumbering: You have disadvantage on all Dexterity checks.
51–60Sickly: You have disadvantage on all Constitution saving throws.
61–63Horrible growth: A large goiter, immobile tendril, or useless extra eye hangs from your face. You have disadvantage on Charisma (Persuasion) checks and advantage on Charisma (Intimidation) checks.
64–66Useless limb: One of your limbs is unusable or missing. If it’s your arm, you can no longer hold anything with two hands, and you can hold only a single object at a time. If it’s your leg, your speed on foot is halved, you must use a cane or crutch to move unless you have a peg leg or other prosthesis, you fall prone after using the Dash action, and you have disadvantage on Dexterity checks made to balance.
67–71Useless eye: One of your eyes is unusable or missing. You have disadvantage on Wisdom (Perception) checks that rely on sight and on ranged attack rolls.
72–76Useless ear: One of your ears is unusable or missing. You have disadvantage on Wisdom (Perception) checks that rely on hearing.
77–84Weakness in Strength: You have disadvantage on all Strength checks and attack rolls that use your Strength modifier.
85–92Weakness in Dexterity: You have disadvantage on all Dexterity checks and attack rolls that use your Dexterity modifier.
93–00Weakness in mind: This affects your Intelligence, Wisdom, or Charisma (equal chances). You have disadvantage on all checks and attack rolls using that ability score.

Created by


Statblock Type

Monster / Creature