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.
Ap/level runes+ sheen = around 50 ap which = +1% to nasus' ulyimare damage.
ghost and cleanse and mercury treads help you stay in the action.
level up spirit fire for creeping fast, but you should note that it might be better to just max siphon first and last hit if you cant get golem buff. maxing siphon will also help you build siphon damage.
this build does massive damage while staying tanky, but the build is very expensive. if your team needs a tank really badly you should hold off on the atmas and trinity, though these two items are important to making nasus a threat.
notice the low magic resist. add in magic res such as force a nature if the team is heavily magic.