Concept art for an illustration for uni. Part of the lore of the Shattered Lands.

The Black Mass is a term used to describe the product of a merging between several lords of the Vaal'Na (it's hard to spell alien words in English). Their total assimilated mass is gathered to a single point before the lords devour and merge themselves, creating a proportionally larger being based on the strength of each lord. Either the hive will be able to compensate for the required energy and control; resulting in a higher form, or the merging will fail and collapse; thus forming the Black Mass.
When this form is created, it is fundamentally unstable. It will attempt to keep its shape but will slowly dissolve into a shapeless mass before breaking up into smaller, lesser forms. The shape depicted is their initial appearance once the failed merging is complete. These things are in the hundreds of feet tall with multiple appendages sprouting from their main bulk. The 'head' is somewhat arbitrary considering the creature can change form, and seems completely aware of it surroundings regardless of the direction the 'head' is facing. It resembles a far more advanced form that can exist stably, although at a fraction of the size. This creature has no known weakness, however they are vulnerable once they collapse into smaller forms. There has only been a single documented case of the Black Mass being destroyed before its collapse, although specifics have not yet been declassified.
When the Black Mass forms on the battlefield, combatants need to be wary of their proximity to the creature. Contact will cause the corruption to spread and will result in assimilation into the creature, increasing its mass. Strategists theorize that if such a mass isn't properly contained, it could cause a chain reaction resulting in a world-ending scenario, thus the Black Mass is colloquially named Consumer of Worlds.