As I have downvoted this guide, I figured it would be nice to at least have the decency to explain why.

No offense, but I wasn't surprised when I checked your profile and found out you're not yet level 30. That's not a problem, but it means you're still learning about how to play certain champs and it is generally not a great idea to write guides for others.

The basic reason why I downvoted is because of your items. Starting with a Sapphire Crystal is ok, getting Sheen asap is what you should do and buying Trinity Force is a must. Apart from that, pretty much all of the items are bad.
- Ionian Boots of Lucidity are completely unnecessary. Yes, you want to have a low cooldown on your Siphoning Strike, that much is true. You achieve this, however, by maxing it asap. It is also possible to get some cooldown reduction in runes (although tanky runes on him really are better) or masteries (though I'm pretty sure it's better to get the experience masteries) or items (I would recommend looking at Frozen Heart for that matter) but it's not necessary. Also, in your case you get 15% from Ionian Boots of Lucidity, 15% from Shurelya's Battlesong, 20% from Morello's Evil Tome, 3% from Sorcery and ~6% from 9x Greater Glyph of Cooldown Reduction. That's a total of 15+15+20+3+6 = 59%. This is impossible, because there is a cooldown reduction cap of 40%. You're wasting gold by buying more cooldown reduction than you can actually have.
- Zeke's Harbinger is a great late game item that you get once you have finished your core build to increase your own attack speed and lifesteal as well as boosting that of teammates. Not necessarily bad, but definitely not one of the first items you should get.
- Morello's Evil Tome is an absolute joke on Nasus. You're a bruiser, meaning you are tanky and deal physical damage. Sure, your Spirit Fire and Fury of the Sands scale with ability power, but that doesn't automatically mean you should buy it. By increasing the damage you deal with Spirit Fire you will kill minions that you could've killed using Siphoning Strike, which is your bread and butter ability and should be farmed as much as possible. Any AP item on Nasus is a waste, seriously.
- Shurelya's Battlesong is an item that is more suited for support champions. You should get an early Philosopher's Stone for the gold that it will give you, but you generally sell this item late game instead of upgrading it.

Overall the build is also not that tanky so that doesn't really make a lot of sense as Nasus should be quite tanky.
I'm sorry but this is a bad build :|

ehhhh am i reading that build right? doesn't really make any sense whatsoever to me...

have you tried the skilling sequence q-w-q-e-q-r-q-e-qe..and so on? If your jungler plans an early gank (for example solotop vs 2, you will have problems there is no word for it, even if you start with mana and not with a Regrowth Pendant), and if you play in mid vs champs like annie, malzahar, kassadin or vladimir, i promise you will regret that because they will prohibit you all farm (you do not buy any hp regen that allows you to take some damage more) and can force you back very often, you just dont get into their reange, especially without Wither

have you tried to buy items like Force of Nature, Frozen Heart and Warmog's Armore in synergy with Atma's Impaler? Try it our, and also try a Sunfire Aegis.

Besides you´re not close to being useless in Teamfights. Your presence deals insame dmg with your Ultimate activated, preferably + Sunfire ;)