Siege Dragon Engine

Gargantuan construct, neutral evil

Armor Class 18 (natural armor)
Hit Points 390 (20d20 + 180)
Speed 50 ft., climb 50 ft.

30 (+10) 12 (+1) 28 (+9) 9 (-1) 20 (+5) 26 (+8)

Damage Immunities poison; bludgeoning, piercing, and slashing from nonmagical attacks that aren’t adamantine
Condition Immunities charmed, exhaustion, frightened, grappled, paralyzed, petrified, poisoned
Senses blindsight 60 ft., darkvision 120 ft., passive Perception 15
Languages understands telepathic commands but can’t speak
Challenge 22 (41,000 XP)


  • Battlements. Maggots and grylluses fight from the narrow battlements on the siege dragon engine’s back, mostly behind arrow slits and other forms of cover that render attacking them directly ineffective until the engine is reduced to 0 hit points and destroyed. At that point, most of the fortifications collapse and any remaining uses of the engine’s Deploy Troops action determine what creatures remain on the dragon’s back; these survivors have only half cover atop the engine’s ruins.
  • Immutable Form. The siege dragon engine is immune to any spell or effect that would alter its form.
  • Legendary Resistance (3/day). If the engine fails a saving throw, it can choose to succeed instead.
  • Magic Resistance. The engine has advantage on saving throws against spells and other magical effects.
  • Magic Weapons. The engine’s weapon attacks are magical.
  • Siege Monster. The engine deals double damage to objects.


  • Multiattack. The siege dragon engine makes three attacks: one bite attack and two claw attacks.
  • Bite. Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 29 (3d12 + 10) bludgeoning damage.
  • Claw. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 23 (3d8 + 10) slashing damage and the target must succeed on a DC 18 Strength saving throw or be knocked prone.
  • Tail. Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage.
  • Deploy Troops (3/day). The engine kneels, depositing one shining gryllus and two simpering maggots (page 209) at an unoccupied location within 20 feet. Roll initiative (+0 modifier) for the group, which has its own turns.
  • Poison Breath (Recharge 6). The engine exhales poison gas in a 120-foot cone. Each creature in that area must make a DC 25 Constitution saving throw, taking 66 (19d6) poison damage on a failed save, or half as much damage on a successful one.
  • Trample. The engine moves up to its speed. It can move through the spaces of Huge and smaller creatures. It can make one claw attack against each creature whose space it enters during this movement. It cannot attack a particular creature more than once during this movement.


The siege dragon engine can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The engine regains spent legendary actions at the start of its turn.

  • Ballista Barrage. The engine’s crew fire mast-sized bolts upon all enemies in a 20-foot-radius sphere centered on a point the engine can see within 120 feet. Each creature in that area must make a DC 16 Dexterity saving throw, taking 16 (3d10) piercing damage on a failed save, or half as much damage on a successful one.
  • Bark Volley. Gryllus crew bark at all creatures in a 10-footradius sphere centered on a point the engine can see within 120 feet. Each creature in the area must succeed on a DC 16 Strength saving throw or take 14 (4d6) thunder damage and be knocked prone.
  • Cage Drop. The engine’s crew drop iron cages on up to two Medium or smaller creatures within 10 feet of the engine. Each creature must succeed on a DC 16 Dexterity saving throw or be restrained by the cage until the cage is opened or lifted. A creature can lift the cage with a successful DC 16 Strength (Athletics) check as an action. The cage’s lock can be opened as an action with a successful DC 16 Dexterity (thieves’ tools) check. The cage door can be attacked and destroyed (AC 19, 18 hp, immunity to poison and psychic damage, resistance to acid damage).
  • Tail Attack. The engine makes a tail attack.


The Underhell understands that any world it invades is likely to attempt the foolish act of defending itself. In order to both break the ranks of its enemies and also minimize their own losses, the Fiend Lords created siege engine dragons. These dragons are golems built from the collected corpses of dragons slain by the forces of the Underhell. These creatures are stripped of any free will and forced to serve the Underhell directly. Once the Underhell has forged these creatures, they provide the foundation for semi-living fortresses.

Mobile Fortresses. The minions of the Underhell build simple battlements, towers, and walls on the backs of dragon engines, allowing these same minions to travel securely across a battlefield. From their defensive position, the forces are able to strike against their enemies with ease. These constant attacks from these forces are already a serious threat, and defending against the Underhell becomes all but an impossibility once an engine attacks directly.

This large, wingless, gray dragon has battlements and siege weapons bristling across its back. Dozens of little fiends crawl about up there, aiming the weapons and taking defensive positions among the battlements.

Section 15: Copyright Notice

Sandy Petersen’s Planet Apocalypse, © 2021, Petersen Games.

This is not the complete section 15 entry - see the full license for this page