Nasus is one of the scariest champions in the game thanks to his chasing potential with his Wither and damage scaling Siphoning Strike. His ultimate is probably one of the biggest things to watch out for as it lets him turret-dive you and come out with a good chunk of your health left. Nasus is essentially a beast who you should try to shut down early game.

THIS IS ONLY A BASIC KICKSTARTING GUIDE SO DONT EXPECT A HUGE IN DEPTH GUIDE. I COVER A LOT OF THE BASICS IN THIS GUIDE BUT IT ISNT VERY DEEP BECAUSE THERE ISNT ONE WAY TO PLAY HIM. THIS IS HOW I LIKE TO PLAY HIM AND IT WORKS VERY WELL FOR ME. PLAY AS YOU LIKE AND MOST OF ALL HAVE FUN!!!! :D

Pros:

Super tanky while also maintaing a high damage output
Great chase down potential
One of the best tower divers in the game

Cons:

Can't escape more than 2 champions
Requires a lot of farming for an effective late game
Focuses the entire laning phase on farming

I grab Life Steal Quintessences for longer presence in lane along with Scaling AD Marks for late game teamfights. I also took Magic Resist and CD Reduc Glyphs and flat Armor Seals to last longer in team fights.

For Masteries I prefer going 0/19/11 instead of 0/21/9 mainly because the lifesteal is more useful for me than extra tankyness. I believe you should capitalize and take advantage of your lifesteal as you not only have a bonus 20% lifesteal at level 16 with your passive, build a Spirit Visage for a good 20% lifesteal,(lets ignore the 20% spell vamp you give your spirit fire and 20% drain for your ult) but you now get an extra 3% from Masteries and 6% from your Runes giving you a total of wait for it... 29% Lifesteal. So by the end of the game you should have at least an average 310 damage on your Siphoning Strike meaning just with that you will be healing a good ~100 health which is pretty good deal. When combined with your ult which steals surrounding enemy health to build up your damage you'll be doing bunches more damage and taking even more health back.

Always start your game off by building Warmogs Armor and Boots of Speed or a Randuinns Omen with Boots. After that you should build a Spirit Visage for an even larger Lifesteal and Spell Vamp. A Frozen Mallet will give you increased Health, Damage and chase down potential. Combine the Frozen Mallet or Trinity Force with Furor Merc Treads, and Wither and you should be getting kill after kill. Especially if they run. By level 12, you should almost never doubt your ability to tower dive and enemy champ especially when at full health and your ult is off cooldown.

Always max your Skills in the same sequence whether you're Jungling Nasus or Laning Nasus. Your Siphoning Strike is your main damaging tool and Wither should help you land one after another. Maxing Wither will greatly decrease their Movement and Attack Speed making you and unstoppable force.

For this specific build you need a certain mentality. If you've ever played Rammus before, you'll want to play similarly. You aren't as tanky as Rammus but you deal a ton of damage meaning you'll be under the tower for shorter periods of time when you tower dive. When you start the game, you and your entire team should be thinking, "Nasus, play passively until the late game". Why? Because early game you shouldn't be recalling a lot meaning you'll spend hopefully half the laning phase off of your first purchases. All you'll be doing is focusing on farming up your Q and getting gold for items late game. Nasus gradually gets stronger as the game progresses, so be PATIENT. Once you get your first major item, boots and Spirit Visage you should be pretty much done with laning phase and already have a decent 130 min. farm on your Q. By then you're free to roam around and make plays happen. Try placing wards in their jungle (blue and red buffs are higher priorities), don't be afraid or hesitate unless you don't know where they are. That usually means they're already at their buffs or ganking another lane. If you ever see them out of place or too far to do anything, ward their jungle to steal buffs and/or prevent ganks or even get a kill. Biggest thing I can't stress enough with Nasus is don't be afraid or hesistate to go aggressive mid and late game. If you're in a 1v1 situation you can probably come out on top, so don't worry! They can't touch you! And if you're not sure you can come out alive, Wither and/or Exhaust them and run away!

During the entire laning phase you should be trying to farm up your Siphoning Strike. This means you should try to keep your lane frozen just barely out of tower range. This will let you safely farm your Siphoning Strike. Throughout this part of the game you should only have to recall maybe 2 or 3 times max before you can push your lane out safely. His early game is very slow so be patient. You'll want to auto-attack minions a lot but don't even think about touching them since you'll probably lose a lot of stacks for your Siphoning Strike. Take your time and Q minions at the last second so that you don't miss 3 bonus damage for your Q. Prioritize using your Q on Cannon Minions as it grants double the damage. So make sure to ONLY Q minions when their health is at the minimum. Laning Nasus you should have at least 250 stacks on your Q, Jungling not so much but you should still focus on Qing the bigger monsters. More on jungling later.

When your team initiates a team fight wait before using Wither as many might want to run afterward if they see they're losing. Your first ability should be Spirit Fire and you should try to set it up a bit towards the back of the enemy line. Once you are in the middle of everything activate use your ult to build up your damage ad drain their health. If they focus you that's good because your ADC and Mages should be able to burst the enemy down quickly. If they don't focus you that's good for them because they're actually playing smart. At that point you should keep an eye on your own back line to protect your own ADC and Mages. If they get to the backline, turn around and kill them. They're most likely a burst AP champion so you should be able to take them down pretty easily. In the middle of a major teamfight you shouldn't try to save your Q, just use it as much as you can since it will deal damage and heal you a buttload. And once again, don't be afraid to dive, you're freaking Nasus, you can tank it all day if you have to. (Not really but almost) The major flaw in Nasus players is that they doubt Nasus' potential to tank turrets. That shouldn't ever happen because you're Nasus. Whenever you use Nasus you should feel very confident of winning. Even if your team feeds. If your team falls behind that means you should try to farm up your Q a litte extra.

Nasus is a beautiful champion and one of my favorites. I prefer Jungling with Nasus but I'm more than happy to lane with him because that means I get to eat more minions with my Q. Nasus is excellent. Some people try to AD or build Crit Nasus but I do terribly with that build because it means I can't tank turrets as much as I would otherwise. The most important thing about playing Nasus (or even any champion in the game) is having the right mentality. If you are playing Nasus, you will most likely be the tank so don't worry about trying to make a few risky plays every once in a while. You can also usually 2v1 most champs with Nasus so don't get scared, they should be scared, especially if you're fed.