Eidolons

Why in the world would you make an item aware of its own existence?
-Random citizen

An Eidolon is a special type of sentient Echo created by binding the spirit or soul of an existing creature to the item in place of constructing a new intelligence from scratch. They have the potential to retain any spellcasting abilities they had in life, and unlike other sentient echoes, gain the ability to concentrate on spells cast through the Echo on your behalf, freeing up your mind to focus on other things.
To become an Eidolon, a creature needs to meet one of the following conditions:

• It must be a willing participant in the binding ritual.
• It must be under the effects of the imprisonment spell (specifically, the minimum containment property).
• It must be bound within a magic circle, and in addition to the Pure Chromatite used to create the Sentience property, must be targeted by (and fail to resist) a planar binding spell. If the creature is not a celestial, elemental, fey, or fiend, it can still be targeted by this effect but has advantage on the saving throw.

Once bound, the creature's physical body (if any) is destroyed as its mind and soul are transferred to the item. An Eidolon has the following traits:

• The Eidolon ceases aging. It may still need some kind of sustenance (regular exposure to sunlight, or the blood of humanoids) but otherwise it no longer needs to eat, drink, or sleep.
• The Eidolon retains its Intelligence, Wisdom, and Charisma scores. These may still be increased by +1 for each Pure Chromatite spent on it, up to a maximum of 24.
• The Eidolon defaults to a sentient item's basic communication methods (emotional telepathy). However, the first time you spend Chromatite on the item to enhance this trait, it regains its ability to speak and read any of the languages it formerly knew in life. After this, it learns languages of your choice and gains telepathy with its wielder as normal.
• The Eidolon's senses are reduced to a sentient item's senses, which can be enhanced normally with Pure Chromatite. However, rather than gaining darkvision at the last step, it regains whatever special senses it had in life (darkvision, blindsense, tremor sense, or true sight) at their original ranges.
• The Eidolon retains any alignment or characteristics it had in life.
• The eidolon gains the spellcasting trait if it didn't already have one. If it does not already have a spellcasting modifier, it may choose one from Intelligence, Wisdom, or Charisma. The Eidolon can learn one cantrip and a number of spells of 5th level or lower equal to its spellcasting modifier, and casts these on your behalf (provided it is willing to do so) without requiring any verbal or material components. Consequently, the Eidolon concentrates on the spell, rather than you. Any spells you attach to the Echo are added to the Eidolon's known spells. In exchange for a Pure Chromatite, the Eidolon can change one spell it knows for another one. Whenever an Eidolon casts a leveled spell with this trait, it cannot do so again until you have completed a long rest (it can still use additional castings from tuned spell options, if any).
• If the creature was not a willing participant in the ritual, roll a d100. If the result is a 10 or less, the Eidolon gains the cursed item property. Create a thread in blame-a-gan or petition-the-voices to hash out the details of this cursed property.
• If the item is ever destroyed, the creature bound to the Eidolon is freed, appearing in a space adjacent to the broken item and reverting to its stats and abilities it had in life. The creature immediately takes 27 (5d10) psychic damage and gains 4 levels of exhaustion - assuming it survives it is otherwise freed. It may or may not be hostile, depending on its disposition towards you.

An Eidolon is under no obligation to be loyal to you, especially if its personal interests run counter to your own. For example, if you bind a devil to your sword, and you have no interest in pursuing devilish ventures yourself, it may choose to try and manipulate circumstances to the best of its abilities to get revenge on you or even attempt to seek its own reincarnation (and subsequent escape from servitude). In any case, the item is otherwise treated as an NPC under your control (or the control of another player you designate).
Naturally, this means don't be a dick and meta game. If you've got an item with a consciousness that demands innocent blood every five minutes and you conveniently "forget" to stab the nice guy next door because the item has great stats, you're going to get warned for meta gaming.