This is a guide on how to support as the champion Kayn.
Tank Kayn requires the user to take the path of the Darkin and commit to peeling and tanking for the team, while providing opportunities to engage.

Tank Kayn first requires a good mindset, from your ADC (if they are a random) and especially from you. Taking this champion in the support role can lead to people reporting you from the offset, which sadly is something you need to deal with. However as long as you are willing to play your best, you can easily show that you are not a troll and are playing to win!

Now the second most important part, USE RHAST! Shadow Assassin may look cool but it is for assassinating, hence the name. Rhast allows you to heal off of abilities and therefore give you sustain to tank in fights.
It makes you more of a bruiser and if these qualities are catered towards, can make you a powerful tank capable of turning around the outcome engagements.

Third, remember that this is not an excuse to take Kayn support, but a new and exciting way to support your ADC and team in all aspects of the game.

Ignite is great vs squishy supports and is underrated for its application of grievous wounds which stops healing.

Flash is Flash, take Flash, use Flash for free kills, love Flash for free kills, hate Flash if enemy uses Flash to escape or kill you! Its the circle of your life, but not Flash's, Flash is eternal!

Exhaust not only heavily slows a target, but also reduces their damage dealt. Remember this if an engaging champion dives your ADC, where in some cases exhausting them is a bad idea and instead exhausting their damage dealer is the good one.

STARTING ITEMS / CONSUMABLES:

Ancient Coin (SAFE OPTION): Take coin vs poke match-ups, as poke can easily ruin your early game leading to greater repercussions.

Relic Shield: Relic is good for sustain in lanes where you have the advantage and is also helpful for your adc early, but of course it is hard to know if u have the advantage at the start of the game and therefore relic can be deemed risky due to poke when using its passive.

Refillable Potion: Buy this if you are behind it saves u repeatedly buying potions allowing u to save for items easier.

Health Potion: Buy 3 at the start and see where the game goes.

Elixir of Iron: When full build buy this to allow for extended survivability and tenacity within fights.

TANK ITEMS:

RESOLVE:

Aftershock allows you to stay on the front line longer allowing teammates to deal more damage and you to peel for a longer period of time.

Demolish is great in this meta as taking turret plates is critical for gaining an advantage over your opponent.

Conditioning allows for extended survivability after laning phase when team fights start breaking out and you do not have the required items to tank effectively.

Overgrowth is good for raking up free hp during laning phase prepairing you for team fights and contesting drake.

INSPIRATION:

Biscuit Delivery allows for extended survivability during the laning phase, especially against poke champs and allows you to focus on buying items rather than potions to stay in lane longer.

Time Warp Tonic can be a life saver early game vs ignites and DOT abilities as the instant hp allows you to outlast them. The added movement speed can also be used to help engage just a tad faster to catch opponents and can also be the factor that saves you in a chase scenario, don't go overbored however as it is only 5% MS.

9 Adaptive allows for a little more damage, which this build lacks.
6 Armor - Good vs AD
6 Magic Resist - Good vs AP

Can be changed where needed depending on opposition comp.