On Rhaast since he is not the drain-tanker we love we can't buy any health items since is a waste
Rhaast: Voltaic > Axiom Ldr
4th :Edge>DDance>Collector
5th:GA>Infinity>DDance
LDR is a great items because you can simply go 1v9 against bruiser or tanks simply 1 taping them
Edge is situational in case of too much cc
LDR just makes Rhaast feel like in prime against Bruisers and Tanks for example against a Cho'Gath i did 1650 with R only
I do go collector and Infinity Edge in games where i have a support with Ardent Censer so i just go W Q basic get kill > Smite > R > Q > baisc and 1 shot em all (Remember can build this only if u got an support which can give you attack speed) ofc if u want it maximum damage
The other items are just mandatory listed
After first 3 items i mostly look at the tab and say maybe i should go Edge cause of CC or very important Serpent Fang for shieald + Executioner Calling for healing(Helps alot!)
SA: Voltaic>Axiom
3th: looking if too much CC> rush Edge, too much shieald heal? Rush Serpent and Executioner
Mostly third item can ofcourse be LDR even as SA if you buy LDR as third you can then melt any tank in game (Mostly on SA you need to be consistent with you Snowball)
4th: can be any from thrid slot
5th: can be any from third slot or GA
As Boots i do go Ionian Boots as SA. Armor or Mr as Rhaast( and Ionian but mostly Armor/Mr)
When it comes to runes i just do go Dark Harvest on both forms just to Flex pick at max
>Sudden Impact Cash Back
>Sixth Sense/Memento Cosmic Insight
>Treasure Hunter Adaptive+Adaptive+Health(NOT SCALING HEALTH!)
Ofcourse Stormraider is a great runes since it offers >Axiom Arcanist > Transcendence > Gathering Storm + Sudden Impact > Ulimate Hunter/Treasure Hunter (Works very well for both forms but i still love more the Dark Harvest burst for ganks)
My gameplay is mostly based on Snowball and this runes help me the most because even if i have 3 kills in the first 5 mins or if i am behind they help me comeback or just use the lead i gather in order to carry any games.
Kayn is a very difficult champion when it comes to building cause he dosen't have a simple route like Syndra has and then point clicks everybody
Surley there are a lot to cover but for me this is what it works the best recently and it helps me get 60%+ winrate on 3 accounts