The Fused Shadow is an ancient relic of the Twili people from before they were banished, created during the Interloper War.  In its base form, it offers some magical abilities, but its power truly shows and grows with the collection of the other 3 separated parts of the Fused Shadow.

You have found Fused Shadow has taken a liking to your powers and attuned to you.  Nothing you do seems to get it to unattune from you and it is stuck on your head (DM discretion on how to remove it), covering one of your eyes.  However, drawing such a minimal amount of magic from your latent power, such that it does not affect your conscious abilities, it allows you to see normal through the helmet from the covered eye.

Eye of Twilight.  The Fused Shadow covering your eye lets you see in magical ways in exchange for some of your power.

As a bonus action, you can spend 1 sorcery point into the Fused Shadow to grant yourself darkvision out to a range of 200 feet in the eye covered by the Fused Shadow. In that radius, you can see in dim light as if it were bright light and in darkness as if it were dim light.  This lasts for 10 minutes.

As a bonus action, you can spend 2 sorcery point into the Fused Shadow to grant yourself Devil's Sight out to a range of 60 feet in the eye covered by the Fused Shadow.  You can see normally in darkness, both magical and nonmagical.  This lasts for 5 minutes.

You can not have both of these ocular powers activated at the same time and must wait for one to run out before you can use either again.

Invisible and Incorporeal.  Fused Shadow knows it stands out.  As an action, at the cost of 5 sorcery points, it can become invisible and incorporeal.  No one can see it, you can see out of your covered eye normally, and you can wear other head coverings, but you can still feel the weight of it on your head.  This ability lasts for an hour, at which time anything in the same space as the helmet disintegrates when it reappears.  The helmet can do this twice per day, recharging 1 use at every twilight (just before sunrise and just after sunset).

Forced Subclass.  When Fused Shadow attunes to you, you are forced to take the Fused Shadow Sorcerer Subclass immediately.  If you already have a Sorcerer subclass, it is replaced completely by the Fused Shadow subclass.  If you are level 20 and none of those levels are in Sorcerer, take 1 level from each class you are and put them into the Fused Shadow subclass.  If you manage to unattune from Fused Shadow, you automatically loose all levels in Sorcerer unless you previously had levels in Sorcerer, in which case you gain the same number of levels from Fused Shadow into the previous Sorcerer subclass.

Sentience. Fused Shadow was a Chaotic Evil artifact with an Intelligence of 12, a Wisdom of 15, and a Charisma of 15, but the thousands of years since its last access to magic to fuel itself, and the thousands of years before that since it was created, has degraded its sentience to near nothing.  Only extreme emotions can nudge it from its stupor, but it only truly reacts to negative emotions.

Should the attuned creature feel such extreme negative emotions, the Fused Shadow may choose to communicate telepathically with the wielder, mostly to egg them on to delve deeper into those emotions, or to pass along the hate and bloodlust its creators infused it with.  It speaks in undercommon (Twili) in a deep, gravely voice, has hearing out to 60 feet, and can sense the presence of creatures in the same distance.

Personality. Fused Shadow holds a hatred for Hylians, as they were the enemy of its creators.  On the rare occasion that Fused Shadow is pulled from its lethargy, it reacts negatively to the presence of any creatures that resemble Hylians (DMs discretion), to the point it floods your head with rage and bloodlust to destroy said creature(s).  Should this happen, you must succeed a DC 15 Wisdom saving throw or be overtaken by the rage until you can be jolted out of it.  You can reroll your check every 30 seconds.

The 4 Pieces (Optional).  The Fused Shadow is split into 4 parts.  You have the first one attuned to you already.  If, along your travels, you come across other pieces, they can be reintegrated back together, though in the minimal power state, the other pieces store in their own pocket dimension.

Each additional piece helps amplify your magics.  The second piece increases your magical damage or healing by 1d of the roll.  The third piece increases your magical damage or healing by 2d more of the roll.  The final piece doubles whatever the current dice number is after the third piece is added.

Example: Acid Splash deals 4d6 at max level.  With the second Fused Shadow piece, it now deals 5d6.  With the third piece, it deals 7d6.  With all 4 pieces, it deals 14d6.

Bringing each piece out costs more sorcery points.  To bring the second piece out costs 3 points, the third, 4 more points, and the final piece, 5 more points.  Any and all pieces can be brought out as a single bonus action.

Each additionally piece drains your energy faster though.  Bringing out the second piece starts a 15 round counter.  Bringing out the third reduces that counter by 5.  You can dismiss the pieces at any time without penalty, but if the counter reaches 0, you gain a level of exhaustion equal to the number of pieces you brought out.

If all 4 pieces are brought out, you are transformed into a huge sized, 6 armed, 1 leg abomination made purely out of twilight magic, other than the Fused Shadow.  The round counter is decreased by 5 and you are blind in all senses except a 120 ft truesight vision you gain when you transform.  You have a 40 ft movement speed and can attack 2 more times than you normally can .  Once you transform, you can not dismiss the Fused Shadow until the counter reaches 0, at which time you revert to your current (True or Imp) form unconscious with 5 levels of exhaustion.

Notes: creature with sorcery points.