This is an icon for the t21 effects for survival. If "Expose Flank" (2p) procs, the aura glows. Similiary, if you get 6 stacks of "In for the Kill" (4p) the aura starts shaking. There's also a counter in the middle telling you how many stacks you are on.
Remember, don't consume the 2p effect unless you have 3 or more stacks of the 4p.