Darkheart

Magical Item, greatsword, requires attunement, Legendary

A sentient, double-edged, slightly translucent, pitch-black greatsword. On its’ hilt is a wide, similarly silhouette colored iris with a faint, red catlike slit for a pupil. It has a +4 bonus to attack and damage rolls. It looks like a three-dimensional shadow. Unlike other greatswords, the Darkheart does not have the Heavy property.  

The One Dangerous Game. Upon touching the Darkheart, it will challenge you to a staring contest, the outcome of which is determined by a Wisdom saving throw. The DC is decided secretly by the DM. If you win, the sword rejects you and gifts you one Inspiration point, and you can either walk away or try again in 24 hours. If you lose, you become the sword’s champion. If your attunement slots are full, the DM decides which item you must break attunement with to accommodate the sword. Your alignment shifts to Evil, and you are granted the following abilities:

• You are considered proficient with the Darkheart while attuned to it.
• You are no longer capable of nor need sleep, and you now go extremely long periods of time without needing to blink.
• You receive premonitions of danger and, unless you are incapacitated, cannot be surprised.
• You cannot be disarmed of the Darkheart for any reason. You can use a bonus action to summon it or make it disappear, during which time you can’t attack with it or cast any of its’ spells.
• Each time you make a weapon attack with the Darkheart, which does Necrotic damage instead of piercing or slashing, you regain hit points equal to the Necrotic damage dealt.
• Spellcasting. The Darkheart contains 50 charges, and regains 4d6+2 charges each day at sunset. While you are holding the sword, you can cast the following spells, requiring no material components: Hold Person (2 charges, +1 for every level above 2nd), Slow (3 charges), Thunder Step (3 charges, +1 for every level above 3rd), Dominate Person (5 charges, +1 for every level above 5th), Harm (6 charges), Eyebite (6 charges), Finger of Death (7 charges), Power Word: Stun (8 charges), Dark Star (8 charges), Dominate Monster (8 charges, +1 for every level above 8th), Feeblemind (8 charges), Blade of Disaster (9 charges), and Power Word: Fall (9 charges). If you cast any of the above spells that require concentration, the Darkheart concentrates on it so you don’t have to. All of the spells use your spell save DC and spell attack bonus. If your character doesn’t normally have spells, your spell save DC becomes 20, and your spell attack modifier +10.
• Additionally, the following spells can be cast without expending charges: Jump, Darkness, Detect Thoughts, Invisibility, and Speak with Dead.

Dark Wish. Once every in-game month, you can use an action to cast the Wish spell, although it may come at a cost: After the immediate casting of your wish, roll a d20. On a 1, the sword casts Feeblemind on you, which you automatically fail the save for, even if you stipulate for that not to happen in your initial wish. If this happens, the Darkheart takes control of your character and you become an NPC under the control of the DM.

Each time you use the Darkheart to cast Wish, the d20 roll that decides whether or not it assumes control of you increases by 1. So on your second wish, a roll of 2 or fewer will enthrall you, and so on.

Ending Attunement. A Wish spell that did not originate from you can be used to unmake the Darkheart if it is within 5 feet of the caster.  Alternatively, a prophesied battle with the wielder of the Darkheart’s sister, the Illumina, may result in it being destroyed; if either of these scenarios happen, you die. Dying while attuned to the Darkheart and being later revived, such as by the Revivify, Resurrection, or True Resurrection spells does not end attunement. The Darkheart may decide to break its attunement with you while you are dead, depending on how likely it believes you will be revived.

Sentience. The Darkheart is a sentient Chaotic Evil weapon with an Intelligence of 24, Wisdom of 10, and a Charisma of 20. It has hearing and Darkvision out to 120 feet. The Darkheart’s voice is a deep, guttural, echoing cacophony of many voices, speaking the same words and sentences as one to form individual speech. Occasionally, only one voice will appear instead of the nearly incomprehensible rumble of thousands of wretched souls. The same voice alone is not usually heard more than once.

Personality. The only things the Darkheart loves are manipulation and cunning. It longs to cause chaos and see adventuring parties torn asunder, regardless of who their enemies are. The sword’s exact origins are not clear, and there is no feasible way to get it to own up to its’ backstory. Whenever the Darkheart is not in attunement with a creature, it becomes agitated and desperate for a champion, which it posits as a symbiotic relationship.