Desolation is a maelstrom of death. It is so powerful it also greatly injures the caster as well. The hex takes a fairly long time to cast, and has a long recharge time - but can lay waste to an entire area of living and undeadenemies caught in the swirling maelstrom of destruction.
Jolly Roger uses Soul Storm, an attack similar to Desolation, when he assaults during Invasions. He can do up to 2,000 points in health damage at a time, with NO cost to himself. It looks like a more powerful version of Wither.