In my opinion, nasus needs as much CDR as possible. This build gives him full CDR so you can up the siphon strike to the point of 3 champs running away from you because of it. This build works best against mainly and AD team. If you go against AP then obviously trade out one of the items like GA for a FoN.

Just remember, keep farming and knocking down towers.

TIP: If you are going against a teemo, get an oracles and use your Q on the mushrooms and get your stacks up.