When Kayn is in terrain near you, you will get a visual warning on the edge of the terrain he's in.
Damage Kayn while he's in Shadow Step to shorten its duration, or apply hard crowd control (stun, charm, knockup, etc.) to end Shadow Step immediately.
Kayn is fairly weak before he has transformed into the Shadow Assassin or Rhaast. Coordinate with your team in the early game to apply pressure on him and delay his transformation.
Reaping Slash's dash speed scales with Kayn's movement speed, allowing it work well in conjunction with Shadow Step. The delay duration of the slash is static however.
Only attacking will immediately end Shadow Step. Using abilities such as Reaping Slash and Blade's Reach will not end the effect, allowing for hastier entrances and poke while in terrain.
Umbral Trespass' damage does not ramp up based on the time spent in the target's body. Remaining will leave you untargetable longer, may make it more difficult for the enemy to use an ability to escape and gives your other abilities a chance to become available again. However, it may also allow the enemy to reposition themselves to your disadvantage. Consider the position of your target and both teams when deciding whether to reactivate early or not.
Death's Dance adds an additional form of sustain from all physical damage for both forms, in addition to some resilience to burst damage. The high amounts of AD also scales his healing when using Shadow Step.
Both of The Black Cleaver's passives aid Kayn in skirmishes. Since Reaping Slash deals two instances of physical damage, a well-aimed proc applies the Cleave passive twice, shredding 8% of an enemy champion's armor instead of 4%. Additionally, the passive movement speed buff is welcoming to Kayn's mobile assassin kit, in particular when used with Shadow Step.
Because Rhaast is reliant on healing to last during teamfights and to duel enemies, it is recommended to buy Grievous Wounds items to have a better chance of fighting him. Keep in mind, however, that Bramble Vest and by extension, Thornmail, are less effective against Rhaast than other healing reliant champions. This is because he heals with his abilities instead of his basic attacks.