Restore health over time based on your missing health.
Offense
+1-10% CDR
Flex
+6 Armor or 8 MR (match-up)
Defense
+15-90 Health
WhyPhase Rush
Non-damaging spells count toward proccing Phase rush so a simple [W]->[E]->Auto will let you hunt down your target and resist slows.
WhyNimbus Cloak
Nimbus Cloak's added mobility when casting [R] empowering you to chase down your target for repeated [Q] procs.
If you're playing into a number of AP threats, Nullifying Orb is also a great option to keep you alive in close fights.
WhyTranscendence
More stacks ASAP. Nasus loves every CDR defensive item and with transcendence you don't have to worry about over-capping.
WhyGathering Storm
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.
WhyDemolish
Nasus is all about opening the gates. Demolish + [Q] means you're shredding towers in no-time.
WhySecond Wind
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.