No reason not to (besides money). Parley applies Phage, IIRC >.>
'This would be my cue to say something snarky, wouldn't it? I'll have to remember that. For the next time you make an idiot of yourself...'
Nasus is sick DPS with Siphon Strike.
I get Vampiric Scepter
Boots of Swiftness
The Rest is Situational
If I get primaried, it is armor. If I am not primaried and need DPS then Inf Edge and Phantom Dancer.
I go with Q first then just keep last hitting until I get about 150 on it. Use spirit fire if the creeps are close to hitting your tower and you cannot kill them all.
Get Golem - Lizard whenever possible on the way to a gank. Open up with a slow, spirit fire, ult, Q Q Q Q Q until dead.
I take exhaust and teleport as my spells.
"If freedom is short of weapons, we must compensate with willpower." - Adolf Hitler
IMO, this is the order of your spell build up:
1 - Siphon Strike
2 - Spirit Fire
3 - Spirit Fire
Fury of the Sands first
Level up Spirit Fire first
Then alternate Wither and Siphon Strike.
Don't EVER go with AP on Nas. Only two of his spells get a bonus from AP, Spirit Fire and Fury of the Sands. Spirit Fire scales by .2*AP, while Fury scales .01*AP. Not really worth it.
The first item you should get is Doran's Shield, IMO. Maybe a Meki Pendant or something or even a Sapphire.
Sheen - This is THE item for Nas. Nasus is definitely what I like to call one of the "Sheen Champions," which includes Udyr, Nidalee, Blitz, among others.
Boots - Berzerker Boots are nice on Nasus and go well with his natural lifesteal. Merc Treads obviously if the opponent stacks stuns and stuff. Ninja Tabi if they have Yis and Tristanas. Boots of Swiftness doesn't really do anything for him since he already has Wither, as is Boots of Mobility since he's not the type to be able to enter and exit combat quickly.
Malady - Malady is really nice. He has decent attack speed to begin with (.999 per sec), and this just improves it and stacks upon his natural lifesteal.
Stark's Fervor - If you are playing more of the tank role on your team, then this is a good alternative to Malady, or you can stack it on top if you have gold. You'll not only help your teammates due to Wither, but give them 25% attack speed and 20% lifesteal. If you stack with Malady, that means you get 45/50/55% lifesteal, and 1.588 attack speed (1.743 with Zerk greaves) The armor reduction is great for Nasus and works fantastically with Siphon Strike, too.
Banshee's Veil - Holy crap you need this item. If you are getting boned by a million stuns on their team, this helps so much. Nasus' best bet for health regen comes from lifesteal. If you can't attack, you are screwed. So get this. If you chose to get Merc Treads, then your magic resist becomes a cool 122.
Trinity Force - You should ultimately upgrade your Sheen into Trinity Force. It just gives you so many benefits, it's perfect for Nasus.
Bloodthirster - This is a fantastic item. Minion kills are easy with SF, and after 40 kills, you get an extra 25% lifesteal and 100 damage. By level 18 with Malady and Stark's, your Lifesteal is now 80% per hit, your attack speed is now 1.588 per sec, and you have 216 attack power that you can use Siphon Strike to hit for another 200 or so.
Sheen and some pair of Boots are the crux of any good Nasus build, and arguably Banshee's Veil. If you are taking more of a tank role, consider the following items...
Frozen Heart - Takes your armor to 177, which is a number that's high enough so that armor penetration from enemy weapons usually bring it down to a bit over 100, which is still a good number. The two most important aspects of it are the fact that it reduces enemy attack speed by 25% and reduces your cooldowns by 25%. You can pop a Siphon Strike along with Sheen's effect almost together and maximize damage with the cooldown. This is probably your best item when up against Yis, Udyrs, Nidalees, etc. Pair it with Banshee's and you should be very difficult to kill.
Frozen Hammer - Gives you great health and some damage to boot. Slowing an enemy with your physical attack is just plain cool after the effect of Wither runs out.
Soul Shroud - The health it gives you is great. Combine with Frozen Heart for the maximum cooldown, and gives your team alot of cooldown.
Spirit Visage - This is kind of nice, and a cheaper alternative to Soul Shroud, giving you almost the maximum cooldown, while giving you magic resist and health regen instead. This might be better for early game more than anything.
Atma's Impaler - Nas gets a ton of HP, and this capitalizes on it, giving him 40 attack power for his base, level 18 HP. With Trinity Force and Atma's Impaler, he gets 66 more attack power. The 45 armor is also quite nice. This is great for when the GAME WON'T END, which means you probably will never get this.
CalmofEmptiness pretty much has it nailed down. I wouldn't get a Dorian's Shield though, I'd just rush for Sheen, but that's me. Many people rush Banshee's Veil as it is a really good item on Nasus, but imo, if you have your ulti/merc treads/cleanse, you don't really need it, but it's still good. Generally team buff items like Stark's and Aegis are great items for Nasus. Every item listed there is good for Nasus. I don't like focusing too much on lifesteal for Nasus though because lifesteal is only really good for farming and Nasus is already good at farming with SS, passive and SF. You can just spend the other item slots on other things Nasus needs like armor/cooldowns/mana.
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Generally team buff items like Stark's and Aegis are great items for Nasus.
I forgot about Aegis, that is a pretty good item on Nasus.
I don't like focusing too much on lifesteal for Nasus though because lifesteal is only really good for farming and Nasus is already good at farming with SS, passive and SF. You can just spend the other item slots on other things Nasus needs like armor/cooldowns/mana.
Eh, I suggested the lifesteal because it's great if you go towerds more of a pure DPS. The 80% lifesteal you get from Bloodthirster, Malady, and Stark's is worth it against other melee DPS. Not only are you reducing their attack speed with Wither, but you're also cutting their armor with Spirit Fire, AND you are healing yourself when you fight them.