This is an Off-Tank build for Nasus, the Curator of the Sands. The build isn't that hard and if you're lucky and/or successful in the early game (Farming of your Siphonic Strike is the most essential and hardest part of Nasus in my opinion), you'll be a force to be reckoned with.

P.S.: If someone could help me getting up some Item-pictures and Mastery Spell-pictures I might get my message across a bit clearer, maybe someone could offer me a helping hand by adding a comment. Thanks for reading!

Although I'm not a level 30 summoner, I like to focus my runes on Armor (Marks of Armor Penetration; Seals on Flat Armor; Glyphs on Flat Magic Resistance and Quitnessences on Flat health). With "flat" I mean that you get the amount of armor/health/magic resistance on level 1 without it rising throughout the levels.

The masteries are again a very personal taste in my eyes. I've chosen to go for a 21-0-9 build, getting an Aggressive Nasus. He might be going down twice in the opening 15 minutes, but after that it'll be unlikely he can be killed on a 1 versus 1, if he is played with a bit of experience (say 10 matches).

Seeing as your Siphonic Strike is vital in the later game stages you'll want to upgrade this as fast as possible. The second skill - which slows your opponents - is initially more useful as an escaping mechanism. In the later game stages it'll be hard for your opponents to run away with this second skill AND an exhaust if needed.
The third skill is only usefull after you've got your Sheen/Trinity Force and I do not advice farming with it as it drains your mana in the beginning way too much. You can use it when it's on level 3-5 for farming purposes, when the farming period has already ended. It's really a better option to farm your Siphonic Strike, because you won't get another chance.

The summoner spells you select is much like the runes a personal choice. I prefer Teleport and Exhaust.
1. Teleport is handy for recalling purposes in early game. You can recall to your base and buy that Health Crystal you're in need of, without having to walk the entire end back, etc.
2. Exhaust is useful to help your allies retreat from an incoming gank or help your allies kill an escaping player. Although Nasus' second skill has a slowing option, it won't last long enough if the opponents buy Magic Resistant-boots and slowing down 2 opponents in one battle is a sure winner.

The item choice really depends on who you're fighting, so reading this paragraph will help you choose wisely.
You start of with a Ruby Crystal, so you can lane longer. While you're laning, you should last-hit minions with your Q-skill (Siphonic Strike). The more kills you make with the Q-skill, the more damage it does (every Q-skill-kill adds an additional 3 attack damage points to the Q-skill itself). Having a bonus of 300 extra on your Q-skill means the difference between a kill and a low hp.
After 7 minutes or something, you'll find yourself out of mana. The next item you'll need is a Mana Crystal. If you got the cash, you should buy your Heart of Gold and Ruby Crystal together.
Then you advance to the shoes, which always is a choice you need to make.
- Mages / Characters with Ability Power? Use the Magic Resistance Shoes
- Fighters / Ranged Physical Attackers? Use the Dodging shoes (Ninja Tabi)
- Want to be quicker in fights? Use the Boots of Swiftness
Next is your Sheen, which is vital to hit more on your Siphonic Strike (which means you can farm minions easier).
From there onwards it's pretty situational but I advice a Trinity Force as an upgrade on your Sheen (for more DPS-stylish Tanking) and upgrading your Heart of Gold to a Randuin's Omen.

If you're going full tank, don't buy an Atma's Impaler, but get an Aegis of the Legion, which gives your allies an aura. Also a decent item for a tank build is a Warmog's Armor, but the cost of these items are really huge, so only consider buying them if you get feed a lot or if you're farming a lot of minions.

This entire build isn't a cheap one and most games are over when you've got your Randuin's Omen - Ninja Tabi - Trinity Force. The other items are really situational (if you die a lot, take HP items; if they're only ability power, get a Force of Nature etcetera).

Pro:
- Decent hitter with his Q-skill
- Decent resistance
- With his ultimate he changes a teamfight
- Good slowing possibilies

Cons:
- Expensive build
- Hard to concentrate on farming minions with his Q-skill (tip: You can also farm the neutral monsters ! - Especially handy in the middle game stage)