It occasionally stacks up to 8 or so, but procs so infrequently that it drops off in 1-3 stacks for the most part. Adds about 1.7k DPS according to Details. The tooltip says it builds up until the target’s “demise” and it has a “high chance” to add a stack, but that seems to translate to: “rarely applies a stack within the 20 second window it is up, as long as you are doing nothing but pressing buttons in front of a target dummy.”
Blizz, pretty please fix this. It took a lot of work to pug Mug’Zee enough times to get it to drop…