Nasus is a champion that is underused in my opinion. Although Nasus is a fairly weak early game champion, he has a very strong late game. Also, some of the recent buffs have allowed him to go many ways. Some people often refer to Nasus as a "tank" only champion, but in my opinion, can be played many ways. I'm a type of person who plays champions a bit different and especially as Nasus, I tend to go more aggressive than defensive. Some may prefer Nasus differently, but this guide is just a general interpretation of how I personally like to use him so no need to like it or anything. Well with that being said, I hope you guys will enjoy :)

Pros
No need to worry about mana early levels (Unless you spam Spirit Fire)
Has a decent lane stability given a passive life steal and potions in case
Good amount of farm is key to late game strength
Most likely will be targeted by some enemies (as long as you don't purchase a Guardian Angel)

Cons
Squishy as a solo laner early game
Opponent that lane with a good amount of range and can harass big-time such as Caitlyn, Heimerdinger, and even to some extent, a Xin Zhao with Exhaust make it tough for Nasus to farm.

Runes are a key aspect to Nasus' early game weakness. Nasus at level one (without the runes and masteries) has only 500 hp and very little armor. Despite the fact, Nasus has a decent amount of attack damage and his physical stats per level are quite good. Therefore, recommended runes for Nasus would be flat health quints Greater Quintessence of Health flat armor penetration Greater Mark of Desolation per level armor seals Greater Seal of Defense and per level magic resistance glyphs Greater Glyph of Scaling Magic Resist. The reason for the per level defense items are to keep Nasus stable in the case of solo laning. I put the extra attack damage marks Greater Mark of Attack Damage just so that farming the points with q and getting last hits early game will be a bit easier.

Summoner's are not as important as other aspects, due to the fact that there are possible alternatives for any case and situation. Despite the fact, one important thing we all should know when using Nasus is that most of the time when your team has a jungler or a roamer, Nasus is a champion most allies see as a solo laner. If you are the type of person who likes to play safe, split push, or backdoor, it might be better to grab Teleport instead of the ignite. I personally enjoy grabbing summoners such as Ignite and Exhaust just so that I can waste my opponent's Flash after heavily damaging him. Wasting enemy flash will allow junglers and roamers to have an easier time ganking the lane.

Nasus is mainly used as a tank, and most people who follow that would usually place points in defense and utility. I'm a bit different. I like to have the points on offense and defense so that it can help Nasus be aggressive.

Siphoning Strike
This is the first ability you would get for Nasus. With this, you can not only deal damage in case of level 1 teamfights, but also start farming it's unique +3 bonus damage (getting last hits) from the first minion wave. There is no need to max this ability first, as it's only purpose is the bonus.

Wither
You would want one point in this ability at level 3 just in case. It can be helpful in times of ganks and at times when you feel like pounding your opponent (it can at times be a tool for escaping faster enemies). Maxing this ability earlier may be helpful for succesful ganks and teamfights given it's stable mana cost, but I prefer a stronger burst over the 3% increased slow given the fact that the duration of the slow is a constant 5 seconds.

Spirit Fire
This is an ability that you would want to max first. At level 4, this ability can wipe out enemy minion waves and can also help you a bunch for quick feeding your q ability. With this maxed out early on, it enables you to not only to push lanes faster (very useful in the absence of an opponent) but also help you weaken the enemies in teamfights.

Fury Of The Sands
This ability is just outright amazing. Not only does it boost Nasus' overall health and stats for a time, but also drains the life of enemies converting damage done into bonus attack damage. Despite the long cooldown of 120 seconds, it lasts for 15 seconds (very long for an ultimate) and has a large AoE radius of similar size compared to Spirit Fire. I would suggest you to level this up whenever you can.

As I said before, there are many ways to build Nasus. One way I build Nasus when laning solo is to first start off with Cloth Armor, 3 health pots (for durability) and a mana pot (just in case I spam Spirit Fire too much early on). An alternative for this is to go either Null-Magic Mantle and two health pots (when against AP heavy opponents), or just go a Doran's Shield. The key to building him early game is to purchase key items that will not only keep him strong and stable so that enemies can't do too much, but also allow him to farm his Siphoning Strike. By purchasing a Philosopher's Stone early on will not only give you a good amount of health and mana regeneration for using abilities more frequently (such as Spirit Fire for clearing minion waves), but will also give you a small gold advantage. Items such as the Phage and Sheen should be rushed for laning advantage, and having a Trinity Force early in is great, but if teamfights are expected to happen earlier, never hesitate on buying an Aegis of the Legion before finishing the Trinity because the passive aura for the Aegis will help your team a lot during the early teamfights. The Frozen Mallet is more like tank based item that will enable you and your team to grab kills easier.

The Core Items

Ninja Tabi / Mercury's Threads
Philosopher's Stone Philosopher's Stone
Trinity Force

Alternative Items

Sight Ward Sight Ward
Reason: If there is an enemy jungler who can gank strong (like an Amumu and Nocturne) or an enemy always missing from a lane, never hesitate on buying these. Not too costly for a champion who will easily farm it back :)

Aegis of the Legion before finishing the Trinity Force
Reason: If the laning phase ends earlier than expected, and if you end up being slightly underfarmed, it might be a good idea to prepare an Aegis to strengthen allies during teamfights.

Warmogs Armor for Frozen Mallet
Reason: If you want more health and health regeneration instead of the slow passive and + 20 atack damage. Both will work good with the Atma's Impaler

Phantom Dancer
Reason: By holding a Phantom Dancer in your inventory, Nasus will have the ability to not only have an easier time fighting 1v1 against an enemy in lane, but also be a very good champion to count on when the team needs someone to backdoor. Nasus is a great champion for backdooring enemy turrets and inhibitors, as Nasus has a strong q ability for heavy damage as well as a natural tanky build (late game) which can be helpful when taking damage from enemy turrets and champions.

Thornmail
Reason: Well, if your opponent is a AD heavy champion and will harass you a load, then go for this item. If you purchase this item, keep in mind that the rest of the build should consider the other areas important to Nasus such as magic resistance, attack damage and health.

Banshee's Veil or Force of Nature
Reason: If your enemy or enemies AP carries become a problem before late game, purchasing either of these items might be beneficial.

Late Game Changes
If you have the extra gold to purchase better items, there are a few items you can possibly replace.

Philosopher's Stone Philosopher's Stone
This item will become close to useless after your core build is complete and you hit level 18. With spare gold in your hands, this item can be switched for a more tank heavy item (such as items with more health to increase the effect of the Atma's or pure defense items such as a Frozen Heart and as mentioned above Force of Nature) or a more damage heavy item such as a Tiamat.

Non-Recommended Purchase Items

Guardian Angel
Reason: Once you purchase a Guardian Angel, you will most likely be the last one to be targeted. Until your team has a sure advantage, I would suggest not to purchase this item.

Early Game

Nasus is fairly weak early game. One way you can prevent his early game weakness it to start off with the right items. A way to improve his early game after that is to use Spirit Fire to your advantage. Not only can this ability help you wipe out minion waves, but can also help you keep enemies away while farming your Siphoning Strike.

Mid Game

Remember that if you're winning the lane and taking solo, don't push too hard because if you do, the ganks will come to you. Nasus is a great pusher, so if you are able to somehow take out the enemy alone or with a gank from a jungler or roamer, you will be able to heavily damage enemy turrets.

Late Game

This is the fun part of the game as Nasus. Nasus is a very good teamfighter as he is a strong solo laner. His Wither can slow an enemy enough to heavily damage them. His DoT (AoE) abilities such as Spirit Fire and Fury of the Sands will be more than decent enough to support the team grab kills as well as a few for yourself. Keep in mind that when you are low health, never hesitate to flash or ghost out of a teamfight. You will probably not have a Guardian Angel so it might be better to be a bit defensive.

The strategy to winning the lane with Nasus is to get a good amount of farm with a good amount of stability. Getting last hits with the q ability Siphoning Strike is very important, as this is Nasus' only burst ability. It gives an additional +3 damage to the ability every kill you get (enemy champions, pets, minions, and neutral monsters included) so I suggest you feed it as much as possible. A good combo during team fights is for you as Nasus to initiate with your Wither(or target an enemy for a team to slow), tank in and hit your e ability Spirit Fire on the ground you stand (remember to target and place where there are the most amount of enemies and/or the center of the teamfight) and use your ultimate Fury of the Sands on top of that. This will enable your team to deal greater damage with your e ability's passive armor reducing ability. With good farm on your q ability Siphoning Strike, there is almost no way your team will lose a teamfight (unless the enemy team is winning, or if your team composition is not very good).

Well, that's all that I got for right now. I hope you guys enjoyed it. If there are any questions or comments, please feel free to post. I would like to know your feedback!