Guardian

Guard allies within 350 units of you, and allies you target with spells for 2.5s. While Guarding, if you or the ally take more than a small amount of damage over the duration of the Guard, both of you gain a shield for 1.5s.

Cooldown: 90 - 40 seconds
Shield: 45 - 120 + 12.5% of your ability power + 8% of your bonus health
Proc Threshold: 90 - 250 postmitigation damage