Dissolve into a dark mist to reduce all damage dealt to you by 75%.
While active, you are also invulnerable to control effects and healing magic, and you gain Major Expedition, increasing your Movement Speed by 40%.
Used to be, it was possible to get it's cost low enough that you'd regen the cost between castings. Combine that with the part where devouring swarm would still heal through it... no I don't know why. And Devouring Swarm could be dragged down to something like 4 ultimate. And you can start to see why this one might have taken a few blows from the nerf bat.