"Upon dealing damage, a ring of frost surrounds you for 3s, increasing Armor and Resistance of allies within by 20%. Upon leaving the ring, gain 20% Agility and your next Basic Attack inflicts 35 Frostbite. [Cooldown: 5s]"
Sources
Passive from Frostbound Regalia Armor, Frostbound Regalia Leggings and Frostbound Regalia Helmet