You need to go shadow assassin. With your passive, you stack orbs. These orbs allow you to change your form after getting enough orbs. Attacking ranged champs give you blue orbs, and melee champions will give you red orbs. If you get more blue orbs than red orbs, then you get shadow assassin form first. You can target ranged champs to get shadow assassin first, but that's lowkey toxic so just hope for the best. The shadow assassin is the blue one on top, and if you already selected one shot runes but you get Rhaast form first, it's worth waiting because the runes and build are way to different for Rhaast.



If you go darkin, you should not go this build at all.

Skirmisher's Sabre - Warrior is the obvious choice on Kayn, it allows for more damage for that one shot. The damage is much better than attack speed, or AP, which won't help. Tankiness wouldn't help you one shot either.

Duskblade is absolutely needed for this build. It helps do extra damage after going through a wall or after using your ult. It helps add more damage to your one shot.

Ghostblade is great for this build. It adds movement speed which helps to get to their backline easier, and the lethality and AD make it a clear choice.

ALMOST ESSENTIAL Guardian Angel is pretty self explanatory. If you do die after getting a kill, you can easily get out after reviving. GA can save team fights, and it's armor and AD is helpful.

GREAT FOR ONE SHOTTING Not many see stormrazor in Kayn Assassin builds, but I use it. It helps one shot with it's extra crit damage, and the AD is nice. Stormrazor + Duskblade just adds so much extra damage.
Last whisper items aren't always needed for Assassin Kayn. If they have a lot of tanks, you shouldn't even be going for a one shot, but if you end up having to, last whisper items are great.
GREAT AGAINST TANKS LDR is good all around just for your normal tanks in the enemy comps.
Mortal Reminder is good against some champions. It provides less armor pen, but it does add grevious wounds. Grevious wound items are good against team comps where the enemies have a lot of life steal, or healing. Ex: Dr. Mundo (you should definitely buy mortal reminder), or champs who buy a lot of lifesteal items

Dark Harvest is the best choice for one shotting. For a true one shot, Dark Harvest is the best rune. Basically how dark harvest works, it stacks damage after you kill monsters, minions, or champions. After getting Dark Harvest stacks, your next attack on an enemy champion will do extra damage. The more stacks, the more damage you do. After getting a lot of stacks, you'll do a crazy amount of damage to one shot.

The other runes are self explanatory. The other domination runes help with damage, relentless hunter help make you faster to assassinate faster which is better than healing, and the sorcery runes give extra AD for the one shot.