Patch 1.79 added regular weekly boss battles which offer one Shim'Zar orb as a free reward and a boss crate for $4,99. This crate contains one legendary card, one Gauntlet Ticket, 3 Shim'Zar orbs and a common crate key. The boss crate is worth around $10 and good value for newer players.
Here you can find a lot of not yet released boss details datamined. Beware of Spoilers!