This wide-brimmed conical hat has an organic appearance, with ribbing on in undersides and a monstrous face on its crown. The hat is immune to game effects that would damage or destroy items with acid such as a black pudding's Pseudopod attack.  While you are wearing it, you gain the following benefits:

• You gain a +2 bonus to your spell save DC and spell attack rolls.
  
• You can use your action try to cast a spell that you have seen but don't know. The spell must have a casting time of 1 action or bonus action, and must be at a level you are able to cast.  When you attempt to cast the spell, you spend a spell slot with a level equal to that of the spell you are attempting to cast and make an Intelligence (Arcana) check.  The DC equals DC of 10 + the spell’s level (a cantrip is a 0th-level spell, and does not require a spell slot).  If you try to cast a spell you has seen used in the past hour, you have advantage on this ability check.  If the check succeeds, the you cast the spell. If the check fails, so does the spell, and the action and spell slot used to cast the spell are wasted.
• Ability checks made to remove the hat against your will have disadvantage.

The hat has a 50% chance of actually being a mimic.  Casting the identify spell on the hat reveals if it is a mimic or not.  If the hat is revealed to be a mimic, how the mimic reacts to being discovered is up to the DM.

Hat Mimic

Tiny monstrosity (shapechanger), neutral

Armor Class 12 (Natural Armor)
Hit Points 5 (2d4)
Speed 0 ft., fly 30 ft. (hover)

Skills Arcana +5, Stealth +5

Damage Immunities acid

Condition Immunities prone

Senses darkvision 60 ft., passive Perception 15

Languages Common, Undercommon

Challenge 1/4 (50 XP)

Shapechanger. The mimic can use its action to polymorph into a conical hat or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only). The mimic adheres to anything that touches it. A Medium or smaller creature adhered to the mimic is also grappled by it (escape DC 10). Ability checks made to escape this grapple have disadvantage.

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary witch's or wizard's hat.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it.

Spellcasting. The mimic is a 4th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). The mimic uses its spell slots to casts spells with its Mimic Spell trait.  The mimic can cast 1st level (4 slots) and 2nd level (3 slots) spells.

Spellcasting Savant. The mimic has a +2 bonus to its spell save DC and spell attack rolls (included in in its Spellcasting).

Mimic Spell.  The mimic can use its action try to cast a spell that it has seen but doesn't know. The spell must have a casting time of 1 action or bonus action, and must be at a level the mimic is able to cast.  When the mimic attempts to cast the spell, it spends a spell slot with a level equal to that of the spell it is attempting to cast and makes an Intelligence (Arcana) check.  The DC equals DC of 10 + the spell’s level (a cantrip is a 0th-level spell, and does not require a spell slot).  If the mimic tries to cast a spell it has seen used in the past hour, it has advantage on this ability check.  If the check succeeds, the mimic casts the spell. If the check fails, so does the spell, and the action and spell slot used to cast the spell are wasted.

Mimic Hat. The mimic can be worn as headwear.  While acting as headwear for another creature the mimic takes no actions of its own.  An attuned wearer can sense what the mimic senses as long as it it wearing the mimic, and share's the mimic's Mimic Spell and Spellcasting Savant traits, and can communicate telepathically with the mimic. The wearer can also command the mimic to cast a spell from the mimic's Spellcasting trait, requiring the spell’s normal casting time, using the mimic's spellcasting DC, and using the mimic's spell slots.

While wearing the mimic, the wearer's spell save DC and spell attack bonus each increase by 2.  Ability check made to remove the mimic from its wearer against their will have disadvantage.

The mimic is immune to game effects that would damage or destroy items with acid such as a black pudding's Corrosive Form trait.  Non-acid game effects that would damage or destroy items deal 1d4 damage to the mimic instead of lower its damage rolls.  If an attack or game effect would reduce the mimic to 0 hit points while it is being worn and its wearer has at least 1 hit point, the mimic drops to 1 hit point instead.  While the mimic has 1 hit point or less the wearer loses all benefits provided by the mimic.  The mimic is considered a destroyed item until its hit points rise above 1.

Actions

Pseudopod. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait. If the target is wearing the mimic, it takes make a smother attack against that creature as a bonus action

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) piercing damage plus 2 (1d4) acid damage.  If the target is wearing the mimic, it takes make a smother attack against that creature as a bonus action.

Smother. Melee Weapon Attack: +5 to hit, reach 0 ft., one creature wearing the mimic. Hit: If the mimic is in object form, the target is subjected to its Adhesive trait.  Until it escapes, the target is blinded and at risk of suffocating. In addition, at the start of each of the target's turns, the target takes 3 (1d4 + 1) bludgeoning damage.

Description

Hat mimics pose as well magical witch hats to make themselves desirable to spellcasters. They seek out caches of treasure, typically belonging by a spellcasting creature to hide among other magical items.  Once picked up by an adventurer, a hat mimic will travel with its new owner.  By night, while the owner sleeps, the mimic will rummage through their belongings in search of food to eat or spells to learn.  If discovered (usually when a new owner attempts to identify it) a hat mimic will often try to negotiate; offering its formidable magical adaptability in service to adventurer as its headwear in exchange for regular payments of food and literature.

Hat mimics typically take the form of a black cone-shaped hat with a wide brim, commonly called a "witch's hat".  They utilize a mimic's innate ability to mimic things they have seen to mimic more than mere objects: they mimic spells.  They have great interest in and affinity for magic, and can often be found lurking in wizards' studies.  For the right price, they are willing to serve those who show skill in the arcane arts.

Notes: Bonus: Spell Attacks, Bonus: Spell Save DC, bard, cleric, druid, sorcerer, warlock, or wizard, Creation, Utility, Shapechanging, Headwear, Sentient