Hi and welcome to my guide on how to play Nasus.
This champion can be alot of things, but his main strength (at least from my POV) is that of hurting stuff bad. Nasus can rack up some serious damage, but in order to really bring the hurts there is three things you need focus on: putting points into Siphoning Strike, utilizing Siphoning Strike correctly and last but not least, acquiring the right items.
So first things first, a break down of Nasus' skills:
Siphoning Strike further refered to as SS Q Active
Nasus strikes his foe, dealing damage and increasing the power of his future Siphoning Strikes if he slays his target.
This is your bread'n'butter move. With five points in SS, with a 9 0 21 build and the right rune setup your looking at 3-4 seconds inbetween two SS's. If you've done a good job scoring last hits and sport somewhat decent items you'll be doing 1-2k damage per SS.
Do the math, and you'll realize what I mean when I say he's a hurt-machine.
Wither W Active
Nasus ages his target, decelerating their movement speed and attack speeds over 5 seconds.
A strong disable with a rather long range. Wither works in a peculiar way, it applies a 32% snare and equally strong attack speed reduction which then increases once every second for 5 seconds. This means that by the end of the duration your target will be pretty slow. Best used for chasing down your target of course, but also a rather good escape tool.
Spirit Fire E Active
Nasus unleashes a delayed spirit flame at a location, dealing damage and reducing the armor of enemies who stand in it.
A targeted Area-Of-Effect spell with somewhat low damage but a rather large radius. If you put this inbetween the first row mobs and the back row mobs you'll hit all of them. Great for "denying" other melee champions when your pushing minions or just harassing. However, due to the armor reduction it's also a big part of the hurting stuff bad repertoire.
Open up with wither, but a Spirit Fire down and start spamming SS.
Avatar of Death R Active
Nasus assumes his true form, the Avatar of Death. While in this form he gains additonal health and attack speed, and drains nearby enemies' max health and converts it into bonus damage for the duration of the skill.
A common misunderstanding is that this skill deals damage. It doesn't, or at least not directly. It lowers the max health of any enemy within range by 6% per second at max level. This means you can't and shan't hunt down anyone outside of melee range with this skill.
This is a true killer in large-scale battles, and a nightmare for tanks. But don't hesitate to use it against lower HP targets, the attack speed increase in itself is hurtful aswell.
Can be used as a last ditch effort for escape, as the 500 extra HP is added ontop of your current HP, thus working similar to a heal.
Life steal passive Passive
Nasus has a passive 10% life-steal.
Max out Siphoning Strike and Wither until you hit level 6, get Avatar of Death, keep sinking points into SS and Wither.
At level 1, if your laning without another good disable or laning on your own get Wither first. Otherwise get SS first.
If you feel that your not in a position to get last hits get Spirit Fire early, though not necessarily more then one point, in order to get some money rolling.
Either Armor Penetration, Critical Strike Rate or Critical Strike Damage. I prefer CSR., though APen can be devestating early on if your going for a early gank.
I'd go with one of the two: Baseline HP or HP per level. The former will give you more stopping power early on, which is always nice as melee. The other will give a slight boost end-game. Your choice.
Reduced Cooldown per level. More SS's, more hurts. Also you get to run around looking like a god more often.
Baseline Cooldown reduction. The Quintessence of Celerity scales nowhere near as good as the glyphs which overtake the Focus glyphs already at level 6.
9 0 21
I can't say for certain if Avatar of Death checks for spell resists so Archaic knowledge may be a wasted point. Other then that, make sure to max out deadliness and sorcery on your way down. Improved Exhaust is always nice, and seeing as there is nothing else worhtwhile you might aswell take it over Smite.
1/1 Improved Ghost
Maxed out Awareness or Expanded Mine, take a pick.
1/1 Blink of an Eye
Maxed out Intelligence, put the last point wherever really.
1/1 Presence of the Master
Reduce your cooldown time, increase your movement speed and acquire some nifty improved summoner spells, all in one build!
This paired with your rune setup will enable you to spam SS like a madman.
Paired with Wither you have some pretty sick disabling tools.
Enables you to chase down your target even better.
Getting locked down? Having problemes sticking on your target? CLEANSE!
Really, take your pick from the above four. Exhaust is somewhat useless for chasing down people if you know how to use wither, but the ability to deny enemy melee damage can be useful. My top pick is Ghost + Cleanse.
Focus your hurt! First and foremost get yourself a pair of nice Bezerker Greaves, the added attack speed adds up nicely with your ulitmate.
Secondly, get Sheen. Yeah, you heard me: Sheen. This is so good for Nasus it's almost broken. You've got a low cooldown, hard-hitting ability with a laughable mana cost.
Activating SS triggers Sheen.
After you've gotten yourself Sheen, start upgrading it into a Trinity Force. I usually go with Sheen > Seal > Phage. Seal enables you to outrun most other champions which makes things a whole lot easier.
Once you've gotten Trinity force it's all about upping your damage. The new Brutalizer is a cheap item, with rather low damage but a nice 10% of your cooldowns and some Armor Penetration. If you can't be bothered build straight to a Infinity Edge or Black Cleaver depending on what kind of team your facing.
If the game isn't over by now get yourself a Banshee's Veil against a caster heavy team or a Bloodthirster and you'll be a beast.
Thanks for your time. Any and all feedback welcome!
Edit: I will try to add a strategy part and how to handle more specific encounters aswell, trying to figure out the kinks myself at the moment.