Pros

Unique Ganking Pathing
Healthy Clear
High Sustain And Tankiness "Rhaast"
High Burst And 1-Shot Potential "Shadow"
Very Mobile With His E

Cons

His True Power Spike Starts after Transforming
Not Good In Early Game
Very Weak If Behind

THE DARKIN SCYTHE "PASSIVE":
Kayn wields an ancient, sentient weapon that battles with him for control. Dealing damage against champions causes them to drops orbs that are automatically collected upon leaving combat, filling a bar above Kayn's portrait.

Once the bar is full, he can choose to either be overwhelmed by Rhaast or expel him to become the Shadow Assassin, based on whether he has damaged more melee or ranged champions, respectively. Once primed, the other option becomes available after 4 minutes.

The transformation can only be performed by interacting with their portrait while on the summoning platform, permanently changing their abilities.

Reaping Slash Q: Kayn dashes forward, dealing 60 / 80 / 100 / 120 / 140 (+ 0.65 per bonus attack damage) physical damage to all enemies he passes through before flourishing his scythe, dealing the same damage again to surrounding enemies. Both instances of Reaping Slash deal 55 bonus damage against monsters.

Blade's Reach W: After a short delay in which he can't perform any other actions, Kayn performs an upwards sweep with his scythe, dealing 90 / 135 / 180 / 225 / 270 (+ 1.3 per bonus attack damage) physical damage to all enemies in a line and slowing them by 60%, decaying over 1.5 seconds.

Shadow Step E: Kayn gains 40% bonus movement speed, is ghosted and can ignore terrain collision for 1.5 seconds. Immobilizing effects will interrupt Shadow Step. Kayn heals himself by 100 / 115 / 130 / 145 / 160 (+ 0.4 per bonus attack damage) upon moving through terrain for the first time and, if he's not in combat with enemy champions, Shadow Step's duration is increased to 7 / 7.5 / 8 / 8.5 / 9 seconds. Entering combat with a champion after the effect is enhanced reduces the remaining duration to 1.5 seconds.

Umbral Trespass R: Passive| Kayn's basic attacks and abilities mark enemy champions for a few seconds. Active: Kayn can activate Umbral Trespass to infest a marked enemy champion for up to 2.5 seconds, revealing them and becoming untargetable for the duration. Reactivating Umbral Trespass ends the effect early. Upon reactivation or at the end of the duration, Kayn wrenches free from their body in the cursor's direction, dealing 150 / 250 / 350 (+ 1.75 per bonus attack damage) physical damage to the target.

Always start with Raptors without a leash as kayn can clear AOE camps So easily, Then your clear path will be as following:
Raptors > Red Buff > Krugs > Scuttle > Blue Buff > Gromp > Wolves..
Gank if there is any pushed lane, if not back, get your Stalker's Blade and do same clear rotation.

Since you can transform once in the game so you have to be very wise in your choice!

Kayn has two different forms he can transform to. If you hit melee champions more you can transform into Rhaast, who is more of a bruiser fighter with high sustain. If you hit ranged champions you can transform into the Shadow Assassin. You can also wait 4 minutes after a transformation has been unlocked if you desire the other transformation instead.

Rhaast form fighter style. I tend to build mostly damage and tankiness and he also provide us with self-healing from his passive as "Rhaast heals for 34.5% - 43% (based on level) of the post-mitigation damage that he deals with spells against enemy champions" so that I can still fight opponents and possibly even kill squishes alone.

Shadow Assassin is an assassin form, wanting to HUNT the squishes of the enemy team. Late game though I feel many champions for example like Leona\ Galio can shut this form down easily because of their high CC.

SO, If you're facing a team comp with 3-4 squishes go for Shadow Assassin form and hunt them! And pick Rhaast against Bruisers\Tank team comps and enjoy the 1v2\1v3!

Will update this guide if anything changed in the upcoming patch so stay tuned!
Thank you very much for reading!
It's my first guide here and i'm going to create a daily guides for jungle and mid lane champs, so your rating will help me a lot to improve!