Hey there, This is the build I'm currently running with Nasus. It allows for you to take a vast amount of damage and still make the kills you want to. The build is very focused about the optimum use of your ultimate when available with your passive carrying you through the early game by keeping you in your lane.

While I'm currently still playing around with the runes I find running Glyph of Shielding can help with the endgame as the build does run primarily armor heavy, Hp seals give you an added base for your hp and allow you to still have a decent hp level endgame. The reason why I suggest the Marks of Malice is because those early game critical strikes can easily turn into kills and also can leave you with an easier time farming, regardless how you set your runes up is personal as it's relative to your play style.

I run 0-27-3 For my masteries as it allows you to hold your own early and mid-game where by end-game you should have your item base. As this build works towards a heavy-tanking Nasus it doesn't worry about the minimal gains within the other trees early talents and focuses on survivability.

I would strongly recommend that if you're following this build not to exclude the early 4 items and I do build them in that order, consumables are a personal choice although if you play it right you shouldn't have a need of either mana or health potions. The ninja tabi allows for a large amount of damage avoidance and with the talent points going towards nimbleness can give a needed speed boost for chasing an enemy or getting away. Trinity Force is VITAL, it greatly enhances the damage of your siphoning strikes while enhancing every other stat for you. Build the Sheen first while either working through the other 2 components according to situation or simply finish the whole thing at once, I normally aim not to leave my lane off the bat until i have the 1700 (Buying regular boots first and then heading to my lane) to finish my ninja tabi and sheen. Aegis is important as it is both a great way to get some magic resist, and armor, but it also increases the damage, armor, and magic resist of you and nearby allies. Aegis can be built after Guardian if you feel the need, again, depending on the situation. The last 2 items are just generic and really can be replaced if you find yourself not needing them, maybe go for life-steal, or an infinity edge can be fun too.

The skills I follow allow you to still farm, make kills and don't leave you mana-hungry. It's important to remember the early game follows around almost a passive farmer role while taking the opportunities for a kill. Focus on last-hitting the creeps as having a largely fed Siphoning strike late game will still make you a huge contender for kills and damage out-put late game.
As long as you are able to last hit the creeps you really do not need spirit fire early or mid game to sustain your item-building and will gain a substantial amount of life back because of your passive. By leveling wither, and used at key points you can prevent an enemy from escaping leading to a kill or chasing down you or an ally. It is very important that you level-up your ultimate at every chance as it is your kill maker, very easily with your associated items can you 2v1 even early game with nothing but your boots, sheen, and your Fury of the Sands, and late game you may find yourself easily 3v1 killing your opponents or keeping them busy while your team arrives for the kill.

A small note when it comes to farming, make sure that you are landing last hits on the creeps, most importantly with your siphoning strike every opportunity. But also, if you can't get at the creeps as your opponents are extremely aggressive? Simply sit back, make sure you get the exp and wait for a chance to kill them, being under-leveled can be horribly painful as you miss out on damage from your siphoning strike and are overall at a disadvantage, more so I would say then economically.

The two summoner spells I run faithfully with Nasus are Exaust and Ghost, they both allow for you to make an escape and coupled with wither no enemy should be escaping you.

Even though by the end you can become a vicious and haughty tank it is important to remember that not only does your team need you, you need them. Work together using your ghost to come quickly to an allies side, lead the charge as you're the one you want taking the hits, and remember that while yes, wither is your "they can't get away" that you can use it to save an allies life as well.

All in all, while I'm relatively new to the game this build has been working quite well for me as you really do become a monster that can take an entire team to bring down, remember that it is not an early-game aggressive build but rather more of a mid to late-game build.

Note: You will notice that while there is some health from a few of the items and runes/masteries this build does not emphasize on large amounts of it, why? Because most games you're going to be dealing with both AD and AP, which means somewhere in there may be a madreds while not always the case there are a number of anti-health stackers whereas if you simply focus on armor/magic resist you wont need that health boost. (For most team fights or group vs you fights your ultimate should be up providing you with a temp health boost anyways)