Rethiarius the Sentinel

Actions

• 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.

Reactions

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)

Technological