Passive: While within 350 units of an allied champion, you raise your Guard. Allied champions you affect with unit-targeted abilities are also Guarded for 3 seconds, regardless of distance.
If you or a Guarded ally would take 50 – 178.53 (based on level) damage within 2.5 seconds or lethal damage from an enemy champion, monster or turret, you both gain a shield for 40 – 162.94 (based on level) (+ 20% of your AP) (+ 6% of your bonus health) for 2 seconds.
Guardian only goes on cooldown when the shield is triggered.