Really interesting find!

Although I don't think it's worth changing masteries just for a slightly faster transformation , will have to try to myself a bit later.

Heya ^^

I have a Masteries section in my guide for both Shadow Assassin and Darkin , Stormraider's is just a middle ground keystone that works to some extent on both , but it's way more potent on Shadow Assassin and kinda bad late game for Darkin.

For Darkin you wanna go Fervor or Courage of the Colossus , but again , I explained those choices in my guide in detail.
Also , every mastery works in every lane , there is no specific to jungle , although you might wanna change some minor masteries depending on match up or lane , for examply you're not gonna get the extra duration on jungle buffs if you're gonna lane.

this guide is for shadow assasin or both?

Heya ^^

Fervor is bad for Shadow Assassin as you'll be looking to burst people down during teamfights , therefore prolonged combat isn't something you aim for as Shadow Assassin and Thunderlord's , while viable , it my opinion is worse than Stormraider's because Stormraider's gives that very useful movement speed so you can finish enemies off or run away after you did , although it comes to personal preference.

I actually wanted to include it in my build but it would have exceeded max CDR , although , as you said , it's an amazing item for him , so if you want to go for a bit more damage and less tankyness you can replace the Warrior enchant with a Cinderhulk one and get Death's Dance instead of Spirit Visage.

Heya ^^

For Shadow Assassin you're going for your jungle Item first , preferably mobility boots second , Youmuu's amd then Black Cleaver as third and fourth respectively. The rest is personal preference.

For Darkin you want to go Jungle Item first , Swiftness Boots or Mobility Boots depending on preference (although you may need the Swiftness ones for teamfights) and then Titanic Hydra. It is preferred to also get Black Cleaver right after and also Spirit Visage because it increases your lifesteal significantly. The rest is up to you to decided during the course of the game and team needs.

Thank you , you're very nice! <3

I prefer Shadow Assassin , but sadly right now it doesn't perform as well as Darkin , or at least when not snowballing , because if you are snowballing Shadow Assassin takes the carry role very hard and starts doing a lot.
Under normal circumstances , Shadow Assassin is a bit worse than Darkin and might need some buffs.

Thanks again for reading my guide and I'm really glad it helped , if you are going for Shadow Assassin you can check my guide for Talon as well as their playstyles have some common ground in some points and it might help you with pulling off an assassin playstyle better! <3
(Sorry for self advertisement , but I really want to get those 2 guides to as many people as possible and perhaps help you guys , means a lot to me :D )