I mean, is it really that bad? Look at a cloak of
ElvenKind or Look at a
Cloak of the Bat which is an existing item:
While wearing this cloak, you have advantage on Dexterity (Stealth) checks. In an area of dim light or Darkness, you can grip the edges of the cloak with both hands and use it to fly at a 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 dim light or Darkness, you lose this flying speed.
While wearing the cloak in an area of dim light or Darkness, you can use your action to cast Polymorph on yourself, transforming into a bat. While you are in the form of the bat, you retain your Intelligence, Wisdom, and Charisma scores. The cloak can't be used this way again until the next dawn.
Now look at the Skulker feat, which would make an excellent cloak with a similar flavour:
-You can try to hide when you are lightly obscured from the creature from which you are hiding.
• When you are hidden from a creature and miss it with a ranged weapon attack, making the attack doesn't reveal your position.
• Dim light doesn’t impose disadvantage on your Wisdom (Perception) checks relying on sight.
That said, I'd be wary, though, of adding two feats to an item, like your Ax of Rage. Although, I do think it's thematic.