The Only Guide That You Will Need For Red Kayn!
I've finally made a spreadsheet that I'm very comfortable with!
Those are the 3 builds that I go for every game and they serve me very well (currently Master 600 LP EUW). Obviously, if your playstyle differs from mine it is absolutely fine to disagree. This is just what works for me and what I enjoy the most!
GL HF!
WHAT ABOUT BOOTS?
You should always buy boots based on the enemy team comp. Prioritize Mercs and Steelcaps, and if you don't need them then you can go for Gluttonous (Omnivamp).
WHY YOU SHOULD NOT BE BUYING THESE ITEMS
Steraks - It gives you mostly HP that you don't really care about as it's a Juggernaut item (Sett, Darius, Garen, Illaoi, Urgot), where Rhaast is a light fighter (Riven, Fiora, Ambessa). It's much better to prioritize Maw, but it can be good in some very niche scenarios (vs True Damage like Sett W, Garen R, Darius R etc.).
Visage - It's an item for tanks that rely on non-damage based healing (think Maokai Passive + Undending, Udyr Awaken W, Mundo P and R), where Rhaast has to deal damage in order to heal up. And with such aggressive AD scaling on your abilities (one of the best if not THE best ones in the game) you want to get as much AD as possible. That's why you buy Maw - it's basically a Magic Resist equivalent of Death's Dance. Also keep in mind that Maw gives you 10% Omnivamp on Shield proc FOR THE REST OF THE COMBAT (not 2 sec, not 5 sec, not 10, you can be fighting for 2 minutes and still benefit from that).
Eclipse - It's a conditional item that you have to play around, where Shojin gives you the initial HP that you need. It's not bad, it's just that it competes with better alternatives.
Voltaic - The build path is just atrocious. The item itself is not bad, but it also competes with better alternatives.
Any tank item - Just don't.