The Dark Heart of Astegon is a magic Artifact once given to The Vile Spawn of Insanity Bohrnag by The Chained God.

Bohrnag was imprisoned once by a demi-god a long time ago until some heroes accidentally set him free but they managed to kill Bohrnag not long after. Now the heart is hidden on the Demi plane Astegon, waiting to be found, giving the attuned creature its powers as Bohrnag had.

The heart communicates emotions to the one holding or touching the heart, if the heart senses creatures within 60 ft it can communicate with those creatures the same way. 

Using the heart as a spellcasting focus. You can cast eldritch blast at will.

While attuned to the artifact you gain: The creature that tries to attune to it will know that they have to replace their own heart with The Dark Heart of Astegon.

Spellcasting. blade of disaster once per dawn using a charge.

Immunity. blinded, charmed, frightened.

Cursed. The heart replaces your own heart, removing and killing the heart by any means kills the wearer. Should the wearer be reduced to 0 hit points then the heart leaves the wearer and can't leave within 120 feet from the wearer or they die. See homebrew Monster sheet "The Dark Heart of Astegon"

Destruction. You gain the form of Insanity from the heart and The Special Purpose "Destroyer" as seen in the table "Special Purpose" section of the Dungeon Master's Guide. 

Form of Insanity. You grow one size larger to a maximum of Huge and your natural weapons and unarmed attacks deal an additional dmg dice, your Hit point maximum is increased by 250 HP your AC is also increased by 2. The voices from the heart turn you Insane, you crave knowledge and power, and you have an urge to destroy every living being and cannot control it. 

Breaking the attunement. Only with a wish spell or by permanent death, although even with a wish spell you will not be able to retain your sane state again, you will forever feel like something is missing in you, a void, an urge to destroy but you can withstand that urge with a daily successful Wisdom Saving throw DC 18.

Notes: Bonus: Armor Class, Bonus: Hit Points, Creature with an intelligence of 5 or higher