Cloak of the Bat

Wondrous Item, Rare (Requires Attunement)

While wearing this cloak, you have

If you have Advantage on a D20 Test, roll two d20s, and use the higher roll. A roll can't be affected by more than one Advantage, and Advantage and Disadvantage on the same roll cancel each other. See also D20 Tests.

on Dexterity (

Dexterity (Stealth) allows you to escape notice by moving quietly and hiding behind things.

) checks. In an area of

An area with Dim Light is Lightly Obscured. See also Lightly Obscured and Exploration.

or

An area of Darkness is Heavily Obscured. See also Heavily Obscured and Exploration.

, you can grip the edges of the cloak and use it to gain a

A Fly Speed can be used to travel through the air. While you have a Fly Speed, you can stay aloft until you land, fall, or die. See also Flying and Speed.

of 40 feet. If you ever fail to grip the cloak's edges while flying in this way, or if you are no longer in

An area with Dim Light is Lightly Obscured. See also Lightly Obscured and Exploration.

or

An area of Darkness is Heavily Obscured. See also Heavily Obscured and Exploration.

, you lose this

A Fly Speed can be used to travel through the air. While you have a Fly Speed, you can stay aloft until you land, fall, or die. See also Flying and Speed.

.

While wearing the cloak in an area of

An area with Dim Light is Lightly Obscured. See also Lightly Obscured and Exploration.

or

An area of Darkness is Heavily Obscured. See also Heavily Obscured and Exploration.

, you can cast Polymorph on yourself, shape-shifting into a Bat. While in that form, you retain your Intelligence, Wisdom, and Charisma scores. The cloak can't be used this way again until the next dawn.

Source: Dungeon Master's Guide, p. 245