Split push, scaling, and opening the gates.
Empowered abilities and resource manipulation
Hitting an enemy champion with 3 attacks or separate abilities grants a burst of MS.
Gain cooldown reduction and max cooldown reduction on your ultimate. This is increased further each time you use your ultimate, up to a maximum.
Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Gain increasing amounts of AD or AP, Adaptive over the course of the game.
+65 health and +10 AP or +6 AD, adaptive
Charge up a powerful attack against a tower while near it.
Restore health over time based on your missing health.
Non-damaging spells count toward proccing Phase rush so a simple [W]->[E]->Auto will let you hunt down your target and resist slows.
More [R]s means more reliable damage on command, and more surviveability.
If you're playing into a number of AP threats, Nullifying Orb is also a great option to keep you alive in close fights.
More stacks ASAP. Nasus loves every CDR defensive item and with transcendence you don't have to worry about over-capping.
Usually when playing Nasus you're looking to come online in the mid to late game. Gathering Storm scales incredibly well along side you while you stack away.
Scorch is a solid alternative if you're looking to harass a bit in the laning phase.
Nasus is all about opening the gates. Demolish + [Q] means you're shredding towers in no-time.
Second Wind is a great sustain rune when playing into difficult match-ups. Like Doran's Shield, it allows you to mitigate a lot of the chip damage you might take.
In easy lanes you can take Revitalize to empower you late game [Q] healing.