Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Take it easy
How I have been playing Nasus is you want to try and farm early amap. Nasus is DECENT early game, but hes no early OMGWTF heroes, so don't be trying for early first bloods or ganking all the time. I know a lot of people play with armor pen runes for red, but really? 8.5 armor pen? Yeah its good in all for early game, but that aint gonna give u **** late game, especially if you build him off tank/tank, thats why I take as much early health + heal.
People also like to play him with some form of MP5 item but try to not waste as much mana that you need, and you should be fine. Don't start by spamming spirit fire. At level 1, its doesn't farm the creeps you need at early game. Once you start getting higher levels @ lvl 4 & 5, it will be killing the creeps much faster and getting the gold you need. Focus your mana for stacking syphon strike early game.
Tri force is a no brainer for Nasus. Early/Mid game, whackin kids is good, plus your ult should give you enough health for team fights so that you dont need to buy early tank items. Get Aegis for your team since this build is for him being off tank. Sunfire is next, this allows more health/aoe dmg in team fights. Banshees for more health/mana/MR and that spell shield is always a great pick up. IFFFFF you are struggling with mana or their team is more magic dmg than AD, I would suggest building banshees over sunfire. Last but not least is Atmas. I personally like getting Atmas over another tank item because adding an extra 77 dmg + 18% Crit late game, with Tri Force + Syphon strike, just absolutely dominates 1v1 battles, along with your ult + sunfire helping in the team fight.
Farming is the key part to Nasus. Usually as an off tank you will be playing bottom lane, so make sure you ALWAYS have a ward up at Dragon. Getting ganked and slowing the farming process down is GAME CHANGING to Nasus, since the items in this build are costly. If possible lane with a support character, or high CC character. Personally I like to lane with Taric, AP! Sion, Zil, or Janna. Once you get Sheen you syphon will be hitting pretty hard so playing with a CC character try to harass like crazy with constant stuns/slows and just running up to them and hitting them a few times, you will live DONT WORRY! Also don't run into team fights UNTIL you are level 6! Try not to team fight until then.
Remember this is an OFF TANK BUILD! Feel free to flame. BTW if you were wondering I am a 1550 ELO player and this has been working well in this area, but once I get past this death trap of huge E-peen kids, I feel Nasus is by far the best off tank in the game. Feel free to flame! :D