When Kayn is worth a pick, when it is worth a ban, and what its kit commits your team to.
Locking Kayn commits your team to a fight pattern: trade repeatedly, survive the return damage, and win the third exchange rather than the first. A draft that cannot support that pattern does not improve because you play well.
At 175 attack range Kayn has to walk into damage to deal it, so every trade begins with a decision about whether you can afford the walk.

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.
Kayn dashes, then slashes. Both deal damage.
Kayn damages and slows targets in a line.
Kayn can walk through terrain.
Kayn hides in an enemy's body, dealing massive damage when he bursts out.
Difficulty 8 of 3, melee, fighter. Those three facts decide how safe this is to pick first, and the melee ones are almost always the riskier blind.
Worth reading even when you are the one playing Kayn: it is the list of what your opponent is being told to do to you.
A peerless practitioner of lethal shadow magic, Shieda Kayn battles to achieve his true destiny—to one day lead the Order of Shadow into a new era of Ionian supremacy. He wields the sentient darkin weapon Rhaast, undeterred by its creeping corruption of his body and mind. There are only two possible outcomes: either Kayn bends the weapon to his will... or the malevolent blade consumes him completely, paving the way for the destruction of all Runeterra.
Full Kayn page — abilities, stats and every skin →