Burst damage shielding and teamfight engage effectiveness.
Durability and crowd control
Guard allies you cast spells on and those that are very nearby. If you or a guarded ally would take damage, you're both hasted and granted a shield.
Whenever you gain a shield, your next basic attack against a champion deals bonus adaptive damage.
After taking damage from an enemy champion, the next 3 spells or attacks you receive deal less damage.
Heals and shields are 5% stronger and increased by an additional 10% on targets below 40% health.
Empowered abilities and resource manipulation
Shortly after using your Ultimate, gain a decaying Movement Speed increase that lasts for 3s and allows you to pass through units.
While above 70% health, gain up to +40 Ability Power or +28 Attack Damage (based on level), Adaptive.
Guardian on Rakan allows you to block a large amount of burst damage with a well-timed [E] and the bonus Movement Speed empowers you to follow that up with a strong counter engage.
Shield Bash on Rakan gives you a decent amount of additional defensive power while your passive is on as well as a bit of additional all-in damage pressure.
Bone Plating is great burst damage protection so that you're able to effectively engage and get out with your life.
Revitalize is great at empowering Rakan's heals from items, Font of Life, and [Q] as well as his passive and [E] shielding.
Nimbus Cloak synergizes fantastically with Rakan's [R] granting you the ability to hard engage teamfights easily.
The extra AP from Absolute Focus is great for sustaining in lane with [Q] poke as well as granting you a bit of extra up-front damage with you engage combo.