Living in the varying depths of Middergloom, these rare dragons occasionally rise to the surface world for reasons known only to them. The younger the dragon, the higher up they are usually found, favouring large chambers in which to spread their large translucent, gloomium-hued wings and fly; using the space to soar through the rifts in the earth’s innards.
Little is known about them given their deep- living, and most specimens that have been observed are ones that have reached the surface. Looking like a conventional dragon in form, but with a longer sinewy neck, body, and tail. Its body is covered in flaking scales, and the skin beneath is hard, covered in weeping pustules, angry sores, bristle-haired growths, and vile- looking tumours. The maw of a Gloomium Dragon is lined with sharp piscine teeth and its head resembles that of a giant demonic fish with large round black orbs for eyes, with the iris revealing the green glow inside. The whole body of the dragon gives off a faint bioluminescence, which increases in intensity when the creature is enraged.
Gloomium dragons are fast on the ground despite their size. They are also capable flyers even though they rarely have the room to stretch their wings in the bowels of the earth.*
Use any dragon as the base for this creature and make the following changes.
Immunity to radiation damage.
Chain Reaction. Each creature within 20 feet radius of the dragon’s space must make a DEX saving throw. On a failure, a target takes [1/2 the dragons breath weapon damage] and is flung up to 20 feet away from the dragon in a random direction and knocked prone. Objects are also damaged by this blast and radiation immunity is considered to be resistance in the case of chain reaction damage. If a thrown target strikes an object, such as a wall or floor, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 15 Dexterity saving throw or take the same damage and be knocked prone. The target is also exposed to radiation and must roll its Radiation Die. If the saving throw is successful, the target takes half the radiation damage and isn’t flung away or knocked prone. The explosion will result in a mushroom cloud and leave a radioactive crater in the blast radius of appropriate size.
Mutagenic Aura. If the base dragon has the Frightful Prescence action it also has a mutagenic aura out to the same distance. All creatures within this aura that are not immue to poison must roll their Radiation Die at the start of its turn each round.
Nuclear Slag (Recharges 5-6). The dragon has its breath weapon replaced with nuclear slag. All the base dragons breath weapon metrics stay the same – size, type, shape etc – but the damage type is changed to radiation. A creature that takes any radiation damage must also roll its Radiation Die per the radiation rules. This breath weapon strips poison resistence by one level each time the target takes damage until the creature become vulnerable to poison damage (radiation is considered poison damage for the purposes of immunites and resistences).
Bloody Breath. The first time during an encounter that the dragon is reduced to half its starting hit points its breath weapon recharges and it may use it immiedelty.
* From The Midderlands Expanded book