Fusaiyama no Akuma (5e Creature)
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Fusaiyama no Akuma
Gargantuan fiend, any chaotic or evil alignment
Saving Throws Str +16, Con +16, Cha +9
Ghost-Fire Fury. At the start of each of its turns, the oni chooses cold or fire. Until the start of its next turn, the oni is immune to the chosen damage type and vulnerable to the other.
Magic Weapons. The oni's weapon attacks are magical.
Multiattack. The oni makes two attacks.
Zhanbatao. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 32 (4d10 + 10) slashing damage plus 7 (2d6) cold or fire damage if the oni is raging (see Mountain Demon Fury).
Kanabo. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 10) bludgeoning damage plus 7 (2d6) cold or fire damage if the oni is raging (see Mountain Demon Fury).
Rock. Melee Weapon Attack: +14 to hit, range 100/400 ft., one target. Hit: 32 (4d10 + 10) bludgeoning damage plus 7 (2d6) cold or fire damage if the oni is raging (see Mountain Demon Fury). If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.
Mountain Demon Breath (Recharge 5-6). The oni exhales energy in a 40-foot cone. Each creature in that area must succeed on a DC 20 Constitution (cold) or Dexterity (fire) saving throw, taking 31 (9d6) damage of the type it is immune to on a failed save, or half as much damage on a successful one.
Mountain Demon Fury (1/Day). The oni chooses cold or fire and flies into a wild rage for 1 minute. For the duration, the oni is immune to the chosen type of damage and vulnerable to the other and can't use its Ghost-Fire Fury. If the oni takes damage of the type it is vulnerable to, it can use a reaction to become immune to it and vulnerable to the other.
The titanic fusaiyama no akuma look just like a gargantuan two-headed mountain oni with one head being that of a blue oni and the other head being a red oni, so these creatures are famous for quarreling amongst each other with the red oni head being more impulsive but expressive and the blue oni head being more logical but repressed. Each head can have an unusual number of horns and eyes, one eye or three, one horn or three for example. One side of their body is dark crimson red and the other a dark midnight blue but running down the middle and all over the body are various purple stripes like a tiger. During combat, these creatures agree to settle their arguments and focus on destroying their enemy on hand. The fusaiyama no akuma have a massive purple fanged maw in their belly and they sometimes enjoy hurling foes they have grabbed into their stomach maw. These are the kings of the mountain oni and it illustrates how an oni can become a powerful rakshasa or asura lord with enough time, murders and other heinous deeds committed to their might. Tribes of oni and bakemono that find one will generally worship it, offering slaughtered game or even sacrificing saru bakemono or yokai to feed these mighty beasts. Armies of bakemono and yamaoni might get slaughtered trying to breach a great wall or castle but they will roar in defiance when the akuma appears and simply reduces the defenders' walls to rubble. Thankfully these massive titans are rare and it takes hundreds of years for regular feral mountain oni to ever get so large.