These dudes are nasty. They are perfect shock troops, who, with a nice bit of damage output and a fairly good AC, are going to be a solid enemy for mid- and even high level groups. A squad of them, lead by something like an Orthon, or protecting some spell caster types, would be vicious - and fun - to deal with, especially as with their Deadly Reach power they can lock down skirmisher types and tanking fighters, allowing their own forces to claim some battlefield supremacy.
Imagine one of these guys carrying a few buffs from their spell casters....ouch.
Anyway, here is my take on the "Rage Devil" - though I prefer the idea of calling them Wrath Daemons or something more arcane, such as Furorodaemon.
Furorodaemon (Rage Devil) – Large Fiend (Devil)
A.C. 19 (Heavy Plate Armour)
Hpts: 171 (18d10+72) [90 - 252]
Proficiency Bonus: +4
Str: 22 (+6) Dex: 15 (+2) Con: 18 (+4) Int: 9 (-1) Wis: 18 (+4) Cha: 11 (+0)
Saving Throws: Constitution +8, Charisma +4
Skills: Athletics +9
Damage Resistances: Fire, Poison; Slashing, Piercing and Bludgeoning from non-magical weapons that are not made from Silver
Condition Immunity: Frightened, Poisoned
Senses: Darkvision 120'; Passive Perception: 14
Language: Local Dialect
CR: 12 (8,400 xp)
Blood Rage: The devil inflicts +7 (2d6) extra damage against enemies who are at half or fewer hit points.
Bloodied Fury: As a Reaction, when it reduces an enemy to half hit points, the devil may make an additional Disrupting Maul attack against them.
Deadly Reach: Enemies that make any move whilst within the devil's reach can trigger opportunity attacks, not just those moving away from it.
Multiattack: Devil makes three Disrupting Maul attacks
Disrupting Maul: Melee Weapon Attack, Reach 10', One Target; +10 to hit; Hit: 30 (3d12+12) Bludgeoning damage and the target looses all Immunities, resistances and damage reduction until the end of the devil's next turn. If the target is reduced to half their total hit points or less, they must make a D.C. 18 Strength or Dexterity saving throw or be knocked prone.
Demonscourge Crush (Recharge 5-6): Melee weapon attack; Reach 10', One Target; +10 to hit; Hit: 51 (6d12+12) Bludgeoning damage and the target is Weakened and looses all Resistances, Immunities and Damage Reductions until they make a D.C. 16 Constitution saving throw (at the end of their turns). Weakened characters suffer disadvantage on strength based attack rolls, ability and skill checks, and inflicts half damage with strength based attacks.
These mini's are awesome! They are also a far better representation of how I think this monster looks than the official art...