Hi, I'm a EUW player called sixkills. I'm a Nasus enthousiast and build making enthousiast since Season 1. My top MMR was plat 2 in season 4, so I'm not a high elo player but I do have a lot of game knowledge. What I like about LoL is being creative in making fun builds, not grinding meta builds in ranked. However, I do think this build has the potential to be the best S12 Nasus build. After all, the build is inspired by studying the playstyle from challenger (Nasus) players. They often put a couple levels into E first before maxxing Q. Nasus who relies too much on Siphoning Strike can be shut out from the game easily by enemy laners and junglers, while Spirit Fire gives Nasus a ton of pressure.

A rule on Nasus is: the harder the matchup, the more points you put in E, Spirit Fire. Against champions who have trouble pushing Nasus out of lane I max Q first, tanks like Malphite or Ornn. Against lane bullies like Irelia, Riven, Darius, Teemo, Sett, Trundle, Renekton, Aatrox or Garen I fully max E first. Against average threats I first put three points in E and then max Q, for example against Illaoi, Mundo, Yorick, Poppy, Shen or Tryndamere. I think knowing when to level Q, E or even W, is one of the key aspects to mastering Nasus. After getting some games in with Nasus you will be able to level up your skills intuitively after a while. Just be mindful that there is freedom in how you level up and how you build. Take the liberty to get creative and it will pay off. Try to read the matchup and the situation and then build and level up accordingly. If the enemy has Yasuo, Yone and Jinx in their team, buy a Randuins omen against the critical strikes, and maybe put a extra level in W if Jinx is fed.

This build is inspired by Xpetu's Shen build for climbing season 12:
https://www.youtube.com/watch?v=wcnlugOMAS0&ab_channel=xPetu

Xpetu uses Frostfire Gauntlet + Demonic Embrace on Shen. That works, but I think this combo is even a better fit for Nasus. Especially when paired with Fimbulwinter. The magical thing is, the frost field AA's from Frostfire Gauntlet actually triggers both the Demonic Embrace burn and the Fimbulwinter shield! These 3 items truly have an amazing synergy on Nasus. First Strike + Spirit Fire makes it extremely easy to reach these core items.
The final 2 items are situational, but most of the time I go Stoneplate and Sterak's because both scale well with MAX HP. Having three items that gives Nasus shields makes for a extra treacherous doge to play against. Against heavy AD teams I buy Randuins, against heavy AP teams I go Spirit Visage. If the enemy team has only one fed member, like a Vayne, I might buy Anathema's chains.

A challenger player who explains the playstyle of E-Max Nasus very well in my opinion is TF Blade:
https://www.youtube.com/watch?v=ndOmc63RF0c&ab_channel=TFBlade

That vid should give a good sense about the playstyle. I use different items though. I start with 2 pots + Tear. I do level up E first. You must try to last hit minions with E, hitting the enemy laner in the process to get First Strike gold, Manaflow Band- and Tear stacks. Do take note when Manaflow Band / First Strike is up.
You can even proc First Strike with your W if you need to! Often when your E is on cooldown, but your First Strike is up, the enemy laner will move towards you to try and take down your First Strike. But then you Wither them to still get the First Strike proc and smack 'em with an AA, a Q and another AA to get more First Strike gold than the Spirit Fire proc would've gave you in the first place.

Rush Frostfire Gauntlet.
This will make you tanky and annoying really fast. True, you will miss out on Sheen. which I admit is painful. but Frostfire Gauntlet also deals damage and besides that also slows, procs both Demonic and Fimbulwinter and is 500 gold cheaper than Divine Sunderer. It's components are all really cheap and will make laning phase very safe.

Even after going back to base just once, most of your mana trouble is over, thanks to the Tear, the Manaflow Band and the Cookies. Soon you can keep Spirit Fire and Wither pretty much constantly on cooldown without running out of mana. That is so ****ing annoying for the enemy laner to deal with actually, constantly getting withered and poked with Spirit Fire. This will wear out the mental of a lot of players. They will start cutting you slack when you try to Q minions. They know they are going to get Spirit Fired or even killed if they don't start giving you stacks. At this point, try to kill less minions with E and more minions with Q. Start stacking.

When you have finished Frostfire Gauntlet, now buy Demonic Embrace to unlock your 4 times burn:
Demonic + Frostfire + Spirit Fire + Fury of the Sands.

You should be able to obtain the core build of Tear, Frostfire Gauntlet, Demonic Embrace and Slightly Magical Boots every game, because it is very easy to get gold when farming with Spirit Fire + First Strike. The boots are free and you have cookies for sustain.

Happy bonking! Thanks for reading.