Among the Raid Hotfixes pushed last night, Blizzard also included a small change to Hellcaller Warlocks which has significant impact to the how the Hero Talent tree plays.
This change limits how often you can generate a stack of Wither through Blackened Soul. At first glance, this doesn't seem like a big change; however, it fundamentally changes how the Hero Talent tree is played. Currently, the gameplay as Hellcaller is to manage your Soul Shards and hover around the Soul Shard cap, only spending to avoid wasting shards.
This gameplay was kept until Blackened Soul started consuming Wither stacks; then, you start spending all your Shards to extend Blackened Soul's Wither stack consumption as long as you can, creating a burst window with it. This window was further empowered by Seeds of Their Demise, which provided the Warlock with a steady stream of Tormented Crescendo or Flashpoint procs during it, depending on their spec.
With this change, this gameplay is heavily impacted, as you can only generate a Wither stack every 5 seconds, allowing only brief extensions to the Blackened Soul consumption window.
Our Affliction Warlock writer, Kalamazi, released a video early this morning about this change and its impact on Warlocks.