Arcus Diabolus: Bolverk

One of the 11 Arch-Enemy Weapons built by Kain in the event a calamity struck. This item has 2 forms, a jet-black gun and an obsidian-hooded cloak. It takes a bonus action to switch forms. It is comprised by many microscopic nanites stored in the user's body. While attuned to this item, your AC is equal to your Dexterity Score while unarmored and you gain a +3 bonus to Saving throws as the nanites pump you with energy, enhancing the user's body. This weapon does not require energy cells, it is powered by the nanites harvesting energy. Alternatively, it can be loaded with Bad News bullets and fired that way. In antimagic areas, it will default to "Cloak Form", only maintaining all AC and movement effects. 

Holographic Bird: When you roll for initiative, you summon a holographic Bird companion, an ethereal, black colored raven which cannot be targeted, is immune to all damage and conditions, can fly through walls and barriers and you hear whatever the bird hears. It has a flying speed of 100ft and functions as your Helpful Snipe. While the Holographic Bird is within 100ft of you, you have advantage on all saving throws as the bird and you perceive on-coming threats and better prepares you for them. You can re-summon it or call it to you as an action.

Bugs: With this item, you can make nanite bugs equal to your Proficiency Bonus. As an action, you can Hand these bugs out to allies and they can attach it to themselves or Plant these bugs on creatures, surfaces or objects. These Bugs are invisible, have a walking and climbing speed of 60ft, use your stealth modifier, take a Bonus Action to command and each bug lasts for 8 hours.
-Handed:  If a creature you know is wearing your Bug, you that creature can communicate telepathically through the Bug whether or not you or they have a common language. The communication is possible over any distance, though it can't extend to other planes of existence. As a bonus action, you can allow yourself to hear everything they hear and vice versa. You also always know the Bugged Person's location but only while the two of you are on the same plane of existence.
-Planted(Creature): You always know the Bugged Person's exact location while the Bug lasts, but only while the two of you are on the same plane of existence. While you have this knowledge, the target can't become hidden from you, and if it's invisible, it gains no benefit from that condition against you. As a bonus action, you can command a bug to suppress the Bugged Target's defenses, they must Succeed on a Constitution Saving throw, on a failure, your attacks ignore their resistances for one minute and one round on a Success. You can communicate telepathically with them and they can only communicate back if you allow it. You hear whatever the bugged person hears and see whatever they see from the bug's point of view. Finally, as a bonus action, you can choose to affect these planted creatures and have the nanite bug affect the planted target's mind, making them unable to perceive you with their eyes for one minute, granting a -10 penalty to their passive and active perception checks made to find you made using their eyes, after this effect wears off, the target is immune for 24 hours. In order for a creature to find a planted Bug, they must Succeed an Investigation Check contested by your Sleight of Hand. 
-Planted(Location): You can choose to see and hear whatever the bug hears or sees in its planted location.

Gun Form:  While in its Gun Form, You have a +3 bonus to attack and damage rolls made with this weapon. This rifle can be stored as microscopic nanotech cells inside the user's body and formed whenever the user calls for it. If the user is separated from the gun, the gun disperses into nanotech and returns to the user. The Gun starts at 100% Charge, losing 2% for every shot made. Once at 0%, it cant fire its energy bullets. When you a 20 on the attack roll, you recharge 1 charge of your Legendary Attack. Hit points lost to this weapon's damage can be regained only through a short or long rest, rather than by regeneration, magic, or any other means.

-Ricochet Shot: Once per attack, when you hit a creature with an attack, you can have the bullet ricochet off them and hit another target within 60ft of the 1st target provided the Bullet has a direct path to another target, if such a target exists, make an attack roll against them, on a hit, that target takes the damage of the original shot. Additionally you can also instead shoot an object, surface or barrier and have the bullet ricochet and hit a target within 60ft from the point of impact, if such a target exists, make an attack roll against them. This Function can be readied.

-Warding Shot: Once per round, you can counterattack when a bugged creature tries to sabotage you. If a bugged creature forces you to make a saving throw, you can use your reaction to make one weapon attack against them. You make this attack immediately before making the saving throw. If your attack hits, your save automatically succeeds, in addition to the attack's normal effects. (Only Capable in "Single Fire" Mode)

-Bugging Shot: As an action, you can fire a shot that will plant a Bug on the enemy. Make an attack roll against that enemy within range, if it hits, they are planted with a nanite bug. This attack deal no damage and does not benefit form any effect that would give it damage. This attack consumes no charge. This Function can be readied. (Only Capable in "Single Fire" Mode)

-Rending Shot: As an action, you can choose to spend 50% of your Charge and make a shot. If the Attack Lands, it multiplies the Damage by 2 (After calculating resistances, immunities and reductions, etc.), Or as an action, you can choose to spend 100% of your Charge and make a shot. If the Attack Lands, it multiplies the Damage by 4 (After calculating resistances, immunities and reductions, etc.). This Function can be readied. (Only Capable in "Single Fire" Mode). You can only use this function only with energy shots.

-Stunning Shot: When you attack, you may declare to do Psychic damage instead. If a target is reduced to 0 hit points from this attack the target is instead reduced to 1 hit point and rendered Unconscious for 1 hour instead. (Only Capable in "Adapting Ammo" Mode)

In Gun form, the item has 6 Sections, each of these sections have 2 modes which can be switched as a Bonus Action:
-Barrel
-Frame
-Body
-Lenses (You gain the benefit of the Lenses Section in both gun and cloak form)
-Ammo
-Fire Rate

Barrel
-Unleashed Mode: When you are hidden from a creature and you miss or hit it with a ranged weapon attack, making the attack reveals your position and creatures around the target have advantage on Wisdom (Perception) checks relying on hearing and sight to find you. You gain an additional +3 bonus to attack rolls.
-Suppressed Mode: When you are hidden from a creature and you miss or hit it with a ranged weapon attack, making the attack doesn't reveal your position and creatures around the target have disadvantage on Wisdom (Perception) checks relying on hearing to find you. You take a -3 penalty to attack rolls.

Frame
-Heavy Frame: While in this mode, You get a penalty 5ft of Movement when wielding the weapon. When you are prone, standing up uses 5 more feet of your movement. You gain a +3 bonus to damage rolls.
-Light Frame: While in this mode, You get a bonus 5ft of Movement when wielding the weapon. When you are prone, standing up uses only 5 feet of your movement. Reduces the dice value in which to land a critical strike by 1.

Body
-Enhanced Chamber:  While in this mode, Whenever you have advantage on an attack roll you can reroll one of the dice once. The weapon gains the Loading Property.
-Rotating Chamber:  While in this mode, the misfire score of the weapon is decreased by 3 and the weapon adds 6 more shots before needing to reload for a total of 8.

Lenses
-Far Sight Lenses: While in this mode, nanotech alter your eyes. Your effective range is Doubled.
-Special Sight Lenses: While in this mode, nanotech alter your eyes. You see invisible creatures and objects as if they were visible, and you can see into the Ethereal Plane. Ethereal creatures and objects appear ghostly and translucent, you notice secret doors hidden by magic, and can see into the Ethereal Plane. You can pick up heat signatures of creatures and you can see normally in darkness, both magical and nonmagical.

Ammo
-Regular: While in this mode, you fire Bad News Bullets. You can change the weapon's damage type between and piercing, bludgeoning and slashing.
-Adapting: While in this mode, you fire bullets using the Charges of the Gun. You can change the weapon's damage type between necrotic, radiant, and psychic.

Fire Rate
-Single Fire: While in this mode, when you take the attack action, you fire one bullet when you pull the trigger.
-Burst Shot: While in this mode, when you take the attack action you fire 2 bullets when you pull the trigger. In this mode, twice the amount of the weapon's damage dice are rolled (An example would be 2d12 per attack becomes 4d12 per attack. All other bonuses remain the same) and each shot takes 4%. The Weapon's Misfire Score Increases by 8.

Legendary Attack (1/Day): Before making an attack, the user can choose to forego the attack roll and choose to hit. (Gun Form Only)

Cloak Form:  While in its Cloak form, the nanotech forms into a cloak that adorns you, in this mode the cloak recharges energy. You gain a +3 bonus to AC, your walking speed increases by 30ft and you are always under the effects of the spell "Pass Without Trace". For every 1 hour you have the cloak form activated, it regains 20% energy for the Gun Form to a maximum of 100% charge. This cloak weapon warns you of danger. While wearing the cloak, you have advantage on initiative rolls. In addition, you can't be surprised, except when incapacitated by something other than nonmagical sleep. The cloak magically awakens you if you are sleeping naturally when combat begins.

While wearing this cloak, you can pull its hood over your head to cause yourself to become invisible. While you are invisible, anything you are carrying or wearing is invisible with you. You become visible when you cease wearing the hood. Pulling the hood up or down requires an action. Deduct the time you are invisible, in increments of 1 minute, from the cloak's maximum duration of 4 hours. After 4 hours of use, the invisibility wears off, recharging every dawn. Should the Feature be active until next dawn and still has at least 1 hour left, it does not recharge.

Your thoughts can't be read by telepathy or other means, unless you allow it. You can present false thoughts by succeeding on a Charisma (Deception) check contested by the mind reader's Wisdom (Insight) check. Additionally, no matter what you say, magic or effects that would determine if you are telling the truth indicates you are being truthful if you so choose, and you can't be compelled to tell the truth by magic or effects.

Legendary Resistance (1/Day): If the user fails a saving throw, they can choose to succeed instead. (Cloak Form Only)

 

It’s up to you to decide whether a character has proficiency with a firearm. Characters in most D&D worlds wouldn’t have such proficiency. During their downtime, characters can use the training rules in the Player’s Handbook to acquire proficiency, assuming that they have enough ammunition to keep the weapons working while mastering their use.

Notes: person matching the biometric scan, Range, Two-Handed, Ammunition (Firearms), Reload, Sap