A strange wild magic has opened a portal to a small demi-plane inhabited by mechanical creatures who are looking to defend their realm.
Intro. This encounter is balanced for four players of 2nd level in D&D 5e.
Background. This encounter occurs in a demi-plane, similar to one created by the spell demiplane, and is inhabited by creatures from the plane Mechanus known as modrons. Modrons are created by the ruling deity of Mechanus who sends thousands of his creatures out once every 289 years to scour the outer planes of the multiverse. On this event, known as The Great March, modrons are found in bounty, some becoming lost, destroyed, or trapped in odd places. Three of these creatures became trapped in a demi-plane after wandering in a permanent doorway created by the spell demiplane. Not long after they ventured in, the doorway out disappeared. Unable to leave the modron’s sentience deteriorated over a century and they all became rouge. They are now bound to the demi-plane and will violently protect it from intruders.
Hook. The demi-plane can be reached through a portal that can be found almost anywhere. Good places are underground, a mages study or any place magic goes wild. The portal itself appears as a 4-foot by 3-foot oblong hole punched through a surface, with a shimmering haze at its edges. The demi-plane beyond can be seen if peered through. Assume there is no risk of the portal closing while the characters are investigating and interacting with it. Alternatively, you can set the encounter in a chamber as opposed to a demi-plane if you think the players may not want to enter a strange portal.
Encounter. If characters venture inside they find a curious chamber lit by a magical skylight shining down warm sunlight. The room is circular in shape with a 20-foot radius and a 9-foot ceiling. The room’s floor appears to be made of smooth stone while its circular walls are of hammered brass etched with decorative runes. Eight 5-foot thick pillars stand around a central dais where three shapes stand: a cube and two inverted pyramids. These shapes are actually one dormant quadrone and two dormant tridrones which all turn to inspect the characters if they are detected. Once modrons spot intruders they attack the characters until down to 10 HP or fewer after which they attempt to flee from combat and out of the portal.
Treasure. Upon the dais is a small copper bowl full of 25gp and 30sp.