You get free boots at 10 min. but you cannot buy boots before then. These boots give additional MS. For each takedown you acquire the boots 30s sooner.
Heals and shields are 5% stronger and increased by an additional 10% on targets below 40% health.
Offense
+1-10% CDR
Flex
+6 Armor or 8 MR (match-up)
Defense
+15-90 Health
WhyUnsealed Spellbook
Unsealed Spellbook's overall utility grants you the ability to adapt to situations where you need more damage for a kill, or defensive power from being killed.
WhyMagical Footwear
Magical Footwear allows you to double-down on that gold you're earning from Klepto and Coin by saving you a bunch of gold so that you're able to invest it into combat stats for the mid game.
WhyBiscuit Delivery
Biscuit Delivery further empowers your ability to out-sustain your lane opponent and helps you survive in machups where you may take a lot of early game damage (Gnar/Rumble).
WhyApproach Velocity
Approach Velocity synergizes incredibly well with Nasus' [W] to chase down and stick to your target.
WhyConditioning
Conditioning makes you into a massive mid game front line tank as it will scale well with any defensive items you buy and continuing scaling into the late game with your infinite scaling damage.
WhyRevitalize
Nasus is a champion who lives and dies by his ability to sustain with his passive. Revitalize massively empower that healing in the mid game where you'll be able to out-duel most other champions.