Kayn

Strengths

Look at the line-up of both your team and the enemy's team when picking your form.
Remember that nearby enemies can see which wall you're in.

Passive · The Darkin Scythe — Kayn wields an ancient weapon and fights Rhaast, the darkin within it, for control. Either the Darkin will triumph, or Kayn will master Rhaast and become the Shadow Assassin. Darkin: Heal for a percentage of spell damage dealt to champions. Shadow Assassin: For the first few seconds in combat with enemy champions, deal bonus damage.
Q · Reaping Slash — Kayn dashes, then slashes. Both deal damage.
W · Blade's Reach — Kayn damages and slows targets in a line.
E · Shadow Step — Kayn can walk through terrain.
Ultimate · Umbral Trespass — Kayn hides in an enemy's body, dealing massive damage when he bursts out.

A suggested 5-lane lineup built around Kayn, based on hero partnerships and role synergy — not win rate.