[Infernal] Weapon - Bleeding Heart

This is the first of three legendary weapons, one for each of the three types of fiends: devils, demons, and a new custom type called agons. I really like the idea that "bloodstone" in demon terms is literally a type of gem with a literal heart inside it.

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Blood constantly drips from the head of this axe, oozed from a perpetually living heart encased in red bloodstone. The owner of the heart is long since forgotten and the trapped demon refuses to reveal whether it belonged to it or some unwilling victim.

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Bleeding Heart
Weapon (greataxe), legendary (requires attunement) 

To attune to this magic axe, you must stab the weapon into your chest and expend half your maximum hit dice. If you don't have the hit dice to spend, you gain 3 levels of exhaustion and cannot attempt to attune again until you finish a long rest. Though this process is normally deadly, the axe magically prevents you from dying. Once you attune to the weapon, you absorb the axe and your maximum hit dice is halved until you end your attunement to it.  

Sentience. Bleeding Heart is a sentient chaotic evil weapon with an Intelligence of 15, a Wisdom of 13, and a Charisma of 18. It has hearing and darkvision out to a range of 120 feet. The greataxe communicates telepathically with its wielder and can read, speak, and understand Abyssal.

A malevolent demon named Draeloh'rr lives inside greataxe. It longs for the destruction of all that is complex and doesn't much care who suffers for it, even its wielder. It whispers subtle words to its wielder speaking as if it were the wielder's thoughts to influence them toward violent delights. If the axe is ever outright refused by the wielder, it can rebel and take precise control of the wielder while the wielder is unconscious, committing acts that would change the wielder's mind. 

Joined Body. While you hold the axe, you have the following benefits: 

• You can read, speak, and write Abyssal. 
• You gain darkvision out to 120 feet if you don't already have it. 
• You are immune to the poisoned condition. 
• You cannot be disarmed of the axe.

Heart of the Black Rift. The axe stays inside your chest and emerges only in battle. When you roll for initiative, the axe handle bursts forth from your chest and you can grab it if you have a free hand. Otherwise, you can grab it later on a subsequent turn. You have a +2 bonus to attack and damage rolls made with the axe and you deal an extra 1d6 poison damage to targets you hit that don't have all their hit points. When combat concludes, the axe automatically returns to your body if you are touching it.

Tainted Blood. The axe has a number of charges equal to half your maximum hit dice and regains expended charges when you finish a long rest. When you hit an organic blooded creature with this axe, you can expend 1 charge to deal an extra 2d6 poison damage to it; if this attack kills a target, you gain 10 temporary hit points.

Corruption Beyond Death. If you are reduced to 0 hit points while attuned to this axe, you fall unconscious but your body doesn't fall prone. Instead, the axe takes control of your body and acts for you on your turn. It does whatever it pleases, usually moving and attacking a random creature or object nearby. You can still fail death saving throws as normal during this time, which lasts until you die or are no longer making death saving throws.