Focus on Cooldown Reduction (for effective Q-farm and almost permanent slow with Wither) and durability (Armor and Magic Resist for lane sustain).

Boots of Mobility help roaming around the map. Enchantment Furor is imho the best choice - as soon as you decide to kill someone (even a half-HP Garen, Udyr or Volibear who can run like hell) you'll just chase and Siphoning Strike them to death.

Iceborn Gauntlet and Hydra work perfectly together - AoE slow AND damage every time you Siphoning Strike!

For even more slow and some Health build Frozen Mallet ;) Also Warmog's Armor is reasonable, but Nasus doesn't need the Health regeneration in late game because of his lifesteal (with Hydra/1LS Quint/ Soul Eater = 34% -> 4 Siphoning Strikes heal you back to full life xD)

With Infinity Edge things start getting crazy!!! Once you tried a well Q-farmed Nasus (+600 or more) with Iceborn Gauntlet, Hydra and deal critical damage, you'll never want to play something else ;)

Focus on Cooldown Reduction (for effective Q-farm and almost permanent slow with Wither) and durability (Armor and Magic Resist for lane sustain).

Boots of Mobility help roaming around the map. Enchantment Furor is imho the best choice - as soon as you decide to kill someone (even a half-HP Garen, Udyr or Volibear who can run like hell) you'll just chase and Siphoning Strike them to death.

Iceborn Gauntlet and Hydra work perfectly together - AoE slow AND damage every time you Siphoning Strike!

For even more slow and some Health build Frozen Mallet ;) Also Warmog's Armor is reasonable, but Nasus doesn't need the Health regeneration in late game because of his lifesteal (with Hydra/1LS Quint/ Soul Eater = 34% -> 4 Siphoning Strikes heal you back to full life xD)

With Infinity Edge things start getting crazy!!! Once you tried a well Q-farmed Nasus (+600 or more) with Iceborn Gauntlet, Hydra and deal critical damage, you'll never want to play something else ;)

Just try to get every last hit with Siphoning Strike!

Don't harass the enemy during early game (except you get a gank)! If he does - don't worry! - you'll steal back the missing HP while farming :D

Try not to push - don't use Spirit Fire on minions! If possible, stay almost under your turret.

Obviously you max your Siphoning Strike first.
Second to max is Wither for some hard slow to kill or escape.
Spirit Fire isn't that useful during laning phase but very nice in fights since it reduces the Armor of all enemies in the circle.
Fury of the Sands is one of the mightiest ults in game! Use it in fights to get almost unkillable or to deal some extra damage while doing Nashor - it's almost spammable ;)