Captain America Shield is a popular piece of utility gear. It provides an effect which restores your prayer based on your damage output, very similar to the way in which the Soulsplit prayer heals your health. The shield is popular in the early game as it costs around 100 times less than the Helicopter Pet which also provides the prayer restore effect, although it should be noted that the pet has a large number of other incredibly useful buffs.
The PVP version of the shield can currently be obtained from Battlegrounds, and the PVM version can be purchased from the Avengers Raid reward shop for 100 reward points.
At the moment, the price of Captain America Shield is roughly: 15-20M DScoins