What is the difference between Darkin Kayn and Shadow Kayn?

Kayn is one of the newest champions to enter the Rift. His character's main feature seems to be his mid-game transformation. I've never played as Kayn myself, but I've seen several teammates and opponents use him.

To better understand him, I viewed his abilities in the client. Unfortunately, the descriptions provided there don't give much information in regards to how his passive actually works.

From what I understand, Kayn will transform into either his Darken form, or his Shadow from at some point during the game. Also, I've seen a teammate mentions that he intended to use Darkin Kayn after seeing the enemy's team composition.

Is one form is better suited for dealing damage while another excels as a tank?

I'd like to know what the differences between Kayn's two forms are, and what triggers his metamorphosis.

Solution 1:

Kayn is unique because he is the only champion that is able to switch their core skillset around to match the enemy team comp after the game has started. Rhaast, the Darkin version of Kayn, focuses on hard CC, healing based on damage dealt, and taking down tanks, while Shadow Assassin focuses on killing squishies very quickly.

Kayn's passive, The Darkin Scythe, is what triggers his transformation. Whenever he damages a ranged champion, he will gain points towards transforming into Shadow Assassin. Whenever he damages a melee champion, he gains points toward the Rhaast transformation. Once enough points have been collected for either form, you can choose to initiate the transformation right away at your base (turning into the form for which you collected the most points). Alternatively, you can opt to wait 4 additional minutes to transform into the version which had less points.

After you transform, your abilities are more or less the same, though they are modified slightly.

Passive: The Darkin Scythe

• Rhaast: Heal for some of the damage you deal to enemy champions with your abilities.

• Shadow Assassin: Deal extra damage for 3 seconds after entering combat. The cooldown for this extra damage is 8 seconds after being out of combat.

Q: Reaping Slash

• Rhaast: Deals additional damage based on the target's maximum health.

W: Blade's Reach

• Rhaast: Knock ups affected enemies.
• Shadow Assassin: Gains additional range and damage. Additionally, you can keep moving while casting.

E: Shadow Step

• Shadow Assassin: Removes slows and increases the movement speed buff you get for traveling through terrain.

R: Umbral Trespass

• Rhaast: Deals additional damage based on the targets max health, and also heals you based on the target's max health.

• Shadow Assassin: Gains increased cast range and also resets your passive.