Arms Band

The Arms Band is a Steel Band which wraps around the upper arm and is embedded with gemstones.
The number of gemstones varies with the Rarity as described in the table below.

When an attuned creature wears the Arms Band they gain access to an extra planar space where weapons can be stored and summoned.
Over the course of a short or long rest the attuned creatures may bind weapons to the band, a band can Bind to a number of weapons equal to the number of Gems it holds. Weapons which are already bound can be unbound during the same rest.

A bound weapon can be summoned from the band or stowed into it as a bonus action. If you attempt to summon another weapon from the arms band the first one conjured is dismissed back into the extra planar space. However swapping one summoned weapon for one stored inside the band may be done freely during a turn excluding during an attack with such a weapon.  (Tip: creatures with a multiattack or extra attack feature can switch weapons between attacks, but cannot choose to change their weapon after rolling to hit on a given attack until that attack has resolved). 

While a weapon is summoned the creature gains any benefits of being attuned to the weapon provided they meet its requirements, but they do not use an attunement slot. They gain no benefits of such a weapon while it is stowed inside the band. 

In addition the Arms Band can hold a number of charges equal to the number of gemstones it contains. When you hit with an attack on a hostile creature using a weapon bound to the band, the band  gains 1 charge. Charges stored in the Arms Band are lost after 10 minutes. When you make an attack with a weapon bound to this band you may expend a number of charges to improve your strike, for each charge expended the attack has an additional +1 to its attack and damage rolls charges, expending charges in this manner must be done before the attack roll is made.