[Infernal] Armor - Bonecage

You can't really have demons without bones! This would fit really well with the Fiendish Mask rangers' whole deal with crafting their armor/accessories out of demon and devil bones. I also knew that bone armor is pretty impractical so now it's a bone that sort of grows around a character. I also like the idea of trading some of your hit dice for temporary hit points.

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The crackling of growing bones can be heard from this armor while in absolute silence, still retaining the vitality of their fiendish origins. These grisly items are usually crafted from slain fiends by veteran rangers of the Fiendish Mask conclave who trade their blood for added protection. They become often more fiend and humanoid as a result.

Edit: Readjusted bone spikes to not require a reaction

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Bonecage
Armor (plate), very rare (requires attunement)

This armor initially appears as jagged shards of fiendish bone. To attune to it, you must stab the bone shard into your abdomen and endure the pain for the duration of the attunement period, during which the bone grows into and around your body, forming hundreds of sharp spurs.

Living Armor. You gain a +1 bonus to AC while you wear this armor. When a creature within 5 feet of you grapples you or hits you with a melee weapon attack, you can deal 1d4 piercing damage to it (no action required). A creature can only take this damage once per turn. Also, whenever you finish a long rest, the armor expends four of your hit dice. If it does, it grows new bone spurs that grant you temporary hit points equal to your level.

Impaled. The armor can't be removed from you while you're attuned to it, and you can't voluntarily end your attunement to it. If you're targeted by a spell that ends a curse, your attunement to the armor ends, and it detaches from you.