Well, fahk you... Because it does. Let me explain

Rylai's crystal scepter doesn't work only on ability based spells as lot of people thinks. It works with every single spell (Well except spells like shield and such...) + AP and Health for free? Damn why not?
Frozen Mallet slow enemy with basics? Blade of the ruined king + Nocturne's passive from "W" (Shroud of darkness - Spellshield) Gives you attack speed over 1.56 so if Nocturne with this build gets to you... All you can do is rage after you die with words "OMFG 1Billion slows"
Trinity Force's new passive is rage which gives you movement speed for 2 seconds each time you land a hit means you have furor so you can buy much better enchant on your boots "Homeguard"
Manamune/Muramana "Psycho you're really mad, please call psychiatry, I have found your lost customer" NOPE... Nocturne is quite mana hungry so getting much more mana with regeneration is really good + getting AD from it? PROFIT

How whole build works together/How to use it?

You use Q on your enemy means he gets slow from "Rylai's crystal scepter" and you get movement speed from rage of "Trinity force" + movement speed from your Q means you're so fast and your enemy is so slow that it's like flashing to your enemy then fight starts... You use E on your enemy means 2 seconds of infinity slows means there's no way he can escape from your range so there's no other way for enemy just to get feared and since you have "Rylai's crystal scepter" and "Trinity force" means you have also some AP so instead of 250 damage you do over 350 which doesn't seem big difference but sometimes it's difference between kill/no kill(or no kill & death if you're greedy or get ganged while chasing). If your enemy still "acts" that he can move, just use "Blade of the ruined king" and he'll be slowed so much that he will walk backwards... Plus BOTRK gives you % damage from enemy health and "Manamune" + if you have already "Muramana" and it's activated means even more damage + you're on trail of your Q you have used on enemy or you already used second one = TONS OF DAMAGE

What this build does simply said?

It does: TONS OF DAMAGE + TANKINESS(Lots of HP) with SUSTAIN(Lifesteal x Attack Speed combo) aaand INFINITE SLOWS(For enemy) and INFINITE MOVEMENT SPEED BUFFS(For you)

PS: if you want to be the biggest scumbag ever even more... Just take Ghost instead of Flash XD but I recommend rather Flash

Leave comments and don't be shy to ask ^~^

Psycho