The Kessell's Spheres of Annihilation is an artifact. Its quality varies from epic to mythic with its rank.
It can be found in the [Lockbox of the Magnificent Emporium].
It is also available from the [Glorious Resurgence Epic Artifacts Pack].
An Uncommon rank version is available at the Tarmalune Trade Bar Merchant.
Damage is reduced for each subsequent hit by the same sphere.