Pros + Relatively Low Skill Floor + High Skill Ceiling + Incredibly Versatile + Absurd Amount of Mobility + High Impact Carry + Cool AF Champ

Cons + Skill Shot Reliant + Difficult To Play From Behind + Struggles A Lot Before Transforming + Hard CC Can Shut You Down + Some Champs Outscale Him Heavily + You Run a Risk Of Becoming Edgelord

Shadow Assassin + Best Mobility In The Game + Huge Outplay potential + Extremely Hard to Lockdown + 100-0 Squishies in One Combo + Can Live Where Others Would Die - Easy To Kill If Caught Out - Isn't The Best Scaler - Struggles Heavily Versus Tanks - Relies on Ult As Escape Mechanic - Zhonya's and GA Exist The Shadow Assassin is a constant threat on the map to the enemy team, his high mobility and amazing burst are a major liability to lives of enemy ADC's, Mages and Enchanters. Through Shadow Step Kayn is always able to come through unexpected angles to one shot enemy carry and escape unscathed due to speed and slow resist ganed through it. His R Umbral Trespass is an excellent escape tool that allows him to reposition after succesful assassination of his target. The upgraded Blade's Reach is a major asset to make use of both when engaging AND disengaging.

Darkin Rhaast + Shreds Through Tanks + Incredibly Hard to Kill + Amazing Teamfighter + Useful CC In Form Of His W + Lategame Beast - Can Get Kited Heavily - Anti-Heal Is A Pain - Relies On Team For Engage - Not The Best Burst - Can Get Popped By Fed ADC's Rhaast is nearly unstopable killing machine, once he achieves his core build. He is an absolute menace to face as immobile bruiser or a tank as his Reaping Slash max %HP DMG is absolutely incredible and it heals him based on the damage dealt thanks to his passive. His version of Blade's Reach provides him with a potent tool of disengaging from unwanted fights or dealing major blows to enemies who stick a bit too close to themselves. Through Umbral Trespass he has an effective 2nd health bar as he heals pretty much to full health once full build by using it.

While choosing form to lock into may seem complicated at first it's actually quite easy and it goes as follows, you take a good honest look at enemy team comp and ask yourself a question - can I reliably one shot >=3 enemy champs?

If the answer to the question is yes then feel free to go The Darkin Scythe: Shadow Assassin if the answer is no then the obvious choice is The Darkin Scythe: Rhaast.

Of course there are some exceptions to the rule which you will learn along the way as some champs while squishy have some gimmicks which make Rhaast a better option (ex. Hecarim, Xin Zhao) , while some while being tankier arent'easy to 1v1 directly so it's better to kite them with extra speed of Shadow Assassin (ex. Sett, Trundle).
Furthermore it is important to take your own teamcomp into account, if your facing champs like Sion or Mundo and the rest of enemy team is squishy you still should consider going Red form in order to lock down the splitpush.

As Kayn 8/10 times you want to commit to your full clear and contesting scuttle on spawn.
The other two scenarios are as follows:

1. After clearing one side of your jungle you notice enemy laner heavily overextended with no escape tools (ex. Viktor mid) and your laner has useful setup tools (ex. Galio mid) you can opt to forgo clearing your other side and lvl 3 gank. While doing this you have to take numerous things into account like: where can enemy jungler be? Can he be strong enough to countergank? Are u risking getting invaded by ganking? Is the kill certain?

2. The enemy jungler is champion even weaker than you (ex. Karthus, Zyra) so you plan a cheeky invade. What I like to do is cheese lvl 1 raptor invade by hiding in the bush nearby, optimally u get vision of enemy jungler before hand so u can reliably do 2 things:
A) Steal his whole red side go to your blue side and then contest/defend your blue side (requires help from your laners in form of vision so extremely risky)
B) Clear his raptors and then smite contest his red buff and try to kill him (high risk-high reward)

Once you get your clear out of the way you can transition into your regural jungle gameplay, as Kayn you have the most unique angles available to you like getting through dragon pit and ganking from behind so use it to your advantage.
Also I know we all love dying for form, but please for the love of god don't make dumb plays just to get your form, IT WILL bite you in the *** the further you climb in your steps.

Once you get your form the mid game starts for you and at the same time the nuance ends, you are a real champion now and should do what a real champion does. Farm hard, focus on objectives and get those takedowns. Good luck champ!