Nasus is a poorly Understood champion because of his situational uses. Most people see him simply as another tanky somewhat DPS'ish character and while this is true and possible it is definitely not the only role he can fill. With a couple of defensive items in the early game combined with his fantastic Life steal passive he can last in the lane a long time and be quite hard to kill. I choose to build Nasus as flat out carry, his survivability is built in and he’s quite capable of being an early game bruiser and a late game carry/rip your team to shreds by himself character. I remember a friend of mine saying "Holy **** this 0/8 Nasus on my team just turned into the second coming of Jesus, wasted the whole team and won in one long push ending 12/8"

My build focuses on large sustained damage so I go with a Critical Strike/Attack Speed build with a Dash of Attack Damage, Defense, and Armor Penetration. One of Nasus's absolute biggest problems is that he is easily kited. It is not fun being the guy getting owned because he can’t get close enough to that Ashe to one shot her 5 bars of health and pick up the kill. So my build focuses on early game move speed boosts for those hard to reach places.

I Choose Flat Armor Penetration runes in the Marks because of how much stronger it makes Nasus's Siphoning Strike and Auto attacks, on a second level since they make his Siphoning Strike stronger it’s easier to last hit with making your creep score rise and your Siphoning Strike get stronger and your Items come faster making you hit harder. In the Seals I choose Armor and in the Glyphs Magic Resist, this allows for Nasus to take some of those pokes and prods in the lane and recover easily with his life steal and still be able to farm the entire time. In the Quintessence’s I also took Armor Penetration for the same reasons as Marks.

Substitutes:
Marks: Have no Substitute, Best bang for your buck.

Seals: You can choose to go with health or health regeneration if you like but I think the armor scales a bit better later on.

Glyphs: Mana regeneration is common here but with my build it’s taken care of so I don’t use them. Cooldown Reduction can help you farm your Siphoning Strike faster but that’s about it.

Quintessence’s: Highly suggest the Armor Penetration but If you like, the early game flat health can make him formidable in the lane.

With masteries I choose the 21/0/9 route taking the point in Archaic Knowledge for his Spirit Fire and Ultimate. The Cooldown Reduction in the 9/0/21 is OK but I prefer the boosted damage he gets from the offensive tree since it makes his Siphoning Strike much easier to farm with. Defensive talents are unnessacary if your runes are set up like I have them.

I pick up a Meki Pendent first and two health potions. I take these because it builds into an early chalice which keeps you full on mana while farming with your Siphoning Strike.

After some farm or a need to go back I grab Boots of Speed and turn my Meki Pendent into a Chalice. Again this early chalice makes all the difference in your Siphoning Strike’s End game damage and will help you farm much faster.

Turn the boots into Mercury Treads or Ninja Tabi based on how the other team is set up and start your Trinity Force with a Zeal. Obviously take Merc treads if the enemy team has High Crowd Control and ninja tabi if they are physically heavy on the damage side. The zeal helps get that move speed up alleviating some of that early game Kiting.

Finish Trinity Force; build Zeal to start your first Phantom Dancer. More move Speed is your friend.

Finish First phantom Dancer, Build Zeal but do not make it a Dancer Yet. You’re holding out on the Dancer so that you can pick up some damage and Crit Damage to make those crits count.

Infinity Edge buying the damage Items First then the Crit Cloak. Damage first because at this point you’re completely dependent on your Siphoning Strike’s damage that you have farmed rather than your base auto attack damage.

Turn Zeal into Phantom Dancer.

Sell Chalice when you can afford a Last whisper. The chalice isn’t nessacary anymore and you can grab blue in three or four hits at this point.

I level my Siphoning Strike as first priority unless I'm being harassed really hard in the lane and then I’ll switch Priorities to Spirit Fire. Other than that it’s pretty standard and the alternatives are not advised.

Ok Exhaust is amazing for when you go toe to toe with any other carry not that you need it. Ghost is a must on Nasus as I said earlier he is easily Kited, the build helps out with this but in the early game this makes a big difference in being kited or not, in the late game it’s even more useful as a sprint to make a push right after an ace or to get out of dodge when you’re in real trouble.

Alternatives:

Cleanse: Good choice against those High Crowd Control teams because is your stunned or slowed and can’t catch anyone your damage output is zero.

Heal: I don’t find this particularly useful but It has its places, its best used in a lane where the enemy is harassing you slowly and the heal will set them back a couple of minutes on that harass.

Flash: It’s nice for an escape or to Ult and hop in the middle of everyone but with the move speed from this build it’s basically limited to hopping over a wall to escape or counter an enemy escape.

Fortify: If Nasus’s tower goes down it makes it harder to farm his Siphoning Strike because he has to farm out of position and becomes vulnerable to gank, this can save that tower but it’s very Situational.

Ignite: Your late game damage will be plenty High enough that nobody can heal through it, it can be useful for early kills but you’re more dependent on creep farm then kills.

Teleport: I have used this on occasion against high harass teams so that I can go to base heal and be back in the lane in seconds so as not to miss out on my Siphoning Strike’s farming.

Do not use:
-Revive
-Clairvoyance
-Clarity
-Rally
-Smite

Pros:
-Early game farm with mana regeneration
-Early game sustainability with runes
-Can get easy hits with Siphoning Strike
-Strong late game carry
-No wasted stats
-Not easily kited or outran

Cons:
-Slow to start
-Farm dependent
-Susceptible to Crowd Control
-Not as tanky

Your Siphoning Strike is your bread and Butter and gains 3 points of damage per kill with it use it to last hit and only last hit, if you won’t kill the creep with it then don’t use it. The only time you should be using it off Cooldown and spamming is when you’re engaged in a fight with enemy champions.

As I said before Nasus Is quite misunderstood and just needs a little love to be an amazing late game carry, My build is set up for farming and sustainability early on and is slow to start, but when you get to that Infinity Edge, you Have achieved carry status and should start taking on the other team's carries when you get into fights.