Greetings, fellow Summoners!
My name is Leon, AKA DaddyAwesomSauce.
I am a pharmacist and avid League of Legends player.
Nasus is one of the champions that I've had pretty good success (>60% winrate on ranked), so I wanted to share with you all.

In a nutshell, Nasus is very tanky champion especially with his R Fury of the Sands.
Better yet, he permanently gains increasing raw damage from Q Siphoning Strike, so he can deal a lot of damage while being tanky.
In order to build that damage, you'll have to deal killing blow with Q Siphoning Strike on minions, jungle monsters, and champions (but most primarily from minions) in order to gain the "Q stacks".
Hence farming is very important for Nasus.
Nasus can be a very strong champion when played right (gaining Q stacks and not feeding your opponent).
Nasus is an immobile tank so he'll be in the front line of the teamfights, absorbing majority of the damage from the opponents.
So you have to make sure that your teammates are helping the teamfight (remember, League of Legends is a team game- the overall strength of the team wins the game).
Thank you for stopping by, and enjoy playing Nasus!

Strengths:
Permanently increasing damage with Q Siphoning Strike
Very tanky
Auto attack reset with Q Siphoning Strike
Great synergy with Trinity Force
Split pushing and objective takedown potential
Built-in lifesteal
BM-ing with Control + 3 spam[nextcol]

Weaknesses:
Immobile
Very dependent on good farming
Single target CC only

Nasus can be a very strong champion because he can build damage from Q Siphoning Strike; he can build very tanky and still deal a lot of damage.
Q Siphoning Strike resets the auto attack, which is good for trading and dealing damage to objectives (primarily towers).
Nasus has a passive Soul Eater where he starts off the game with 12% lifesteal, which is great for lane sustain and also stay longer in the teamfight.
Q Siphoning Strike applies the Trinity Force bonus damage, which is a great synergy. All the damage can be dealt to the objectives including towers and inhibitors, which makes him an objective destroying machine (especially with Demolish).

Nasus is immobile so you'll have hard time reaching to the primary target (squishies in the backline).
Despite how strong Nasus can be, your teammates need to play at least on-par or otherwise, you'll be a sandbag.
Not much CC compared to other tanks like Ornn and Sejuani; but his W Wither is pretty strong as a slow spell.
Lastly, last hitting minions with Q Siphoning Strike is very important, so he is vulnerable to farm denies.

Teleport is a good standard summoner spell for top lane.
You'll miss less farms by saving the time you have to walk to top lane (in early game).
In mid-late game, you can clear lane if the waves are pushing towards your tower, then use TP to join your team.
Or if the minion waves are pushing towards opponent's tower and your opponents are way from the tower, you can use TP to the wave to takedown the tower (which Nasus can do so quickly with his Q Siphoning Strike, Trinity Force, and Demolish.

Flash to for escaping or securing kills.
W Wither is a great spell for that purpose too.
In order to secure a kill, you can Flash to your opponent, use W Wither, or in order to escape, you can use W Wither, then Flash.

Ghost can be picked instead of Flash, but I personally never really do that.

Keystone
Grasp of the Undying is pretty boring keystone, but probably the most reliable choice for Nasus.
This rune procs every 4 seconds while in combat and has 3 components:
1) Bonus damage based on your max health
2) Healing based on your max health
3) Permanently gain 5 bonus max health

1) and 2) are good for trading with your lane opponent and lane sustaining.
The effect of 3) is minuscule but can stack up and bonus health is a great stat on Nasus

Second Line
Demolish without a doubt. I don't understand why this rune is not a keystone. That's how great rune is.
Objective control is very important; in my opinion, taking down towers can be worth dying for.
If you die, you give out the kill gold to your opponent(s) and miss farms and levels, but you just respawn.
Towers, on the other hand, once it's destroyed, it's permanently destroyed.
The gold pay out is nice, especially when you take down a tower before your opponents do.
Taking out turret plates can give you up to 750 gold (150 each if you take them down solo, for 5 plates).
Hence, early game tower siege can give you a lot of gold advantage.
Demolish deals a ton of damage to the tower based on your max health (which is high for Nasus since he is a tank) after 3 second charge.
Combined with Demolish, auto attack reset and bonus damage with Q Siphoning Strike, and bonus damage from Sheen or Trinity Force, Nasus is one of the best champion to take out turrets (and inhibitors).

Third Line
Any of the runes work.
Bone Plating is good if you're going for full trading, as it reduces up to 3 incoming damage from an opposing champion (note that this effect only lasts 1.5 seconds and has 45 second cooldown).
Second Wind gives you a lane sustain if you're being poked.
Conditioning gives you additional armor and magic resist, but you have to wait 10 minutes to see the effect. For Nasus, I personally prefer something that works from the beginning of the game ( Bone Plating or Second Wind.

Fourth Line
Overgrowth is my choice.
The effect isn't too big but extra health is a great stat on Nasus.
Other 2 runes in this line aren't very good on Nasus either.

Secondary Tree
My choices are Legend: Bloodline and Coup de Grace.
Legend: Bloodline gives you up to 12% lifesteal (better lane and teamfight sustain) Coup de Grace can increase your damage against low health opponents (the threshold is <40% of max health). This could be substantial on Q Siphoning Strike and could help you deal a killing champion blow with Q (which will give you 12 Q stacks).
From this line, you can also consider Legend: Tenacity and Last Stand.

You could also consider the Inspiration line as a secondary.
Magical Footwear, Biscuit Delivery, and Cosmic Insight are good options.

Bonus Stats
I like to take the CDR bonus stat. Then 2 armor or 2 magic resist bonus stats depending on AD or AP matchup.

Passive: Soul Eater
Great passive, as you start the game with 12% lifesteal built in to you.
If you are trading pokes or being harassed by your lane opponent, you can heal off of minions.
This gives you a lane sustain advantage.
The % is increased by 6% every 6 levels, to 24%.
This healing applies to the bonus damage from Q Siphoning Strike and Trinity Force.
Combined with Legend: Bloodline (up to 12% additional lifesteal), you'll have a significant sustain to stay in the teamfight (even better if you have Spirit Visage, on top of your tankiness to stay alive.

Q Siphoning Strike
This is a signature move of Nasus, where bashes his opponent's skull with his empowered scepter.
This ability is activated when Q is pressed and applies to the next basic attack.
The empowered attack deals bonus damage based on the ability level and the number of "Q stacks"; it also has slightly higher range than the basic attack.

The Q stacks are obtained by dealing killing blows to the minions, jungle monsters, and opposing champions, WITH Q Siphoning Strike
But these stacks comes primarily from minions.
Hence, the laning phase is very important for Nasus, since you'll want to obtain as many Q stacks as possible.

Nasus gains 3 bonus damage on Q Siphoning Strike for each Q stack.
However, Q stacks from the Cannon Minions, large jungle monsters, and opposing champions gives 12 bonus damage instead.
Hence, the Cannon Minions are double important to last hit (with Q whenever possible) for the higher bonus damage and gold.

Q Siphoning Strike activates the Spellbalde bonus damage from Sheen and Trinity Force, so it synergizes very well.
Spellblade bonus damage helps with last hitting and Q stacking, as well as trading pokes with your opposing laner.
Both Q Siphoning Strike and Spellblade damage are applied to the turret, which makes Nasus a great tower breaker.

Another caveat is that Q Siphoning Strike resets the auto attack.
It means when you auto attack any target then press Q, the empowered attack immediately becomes available.
This technique can be used for Q stacking (get the minion health low enough with auto attacks then press Q for last hit), trading pokes (auto attack an opponent then press Q for quick 2 attacks), and taking towers.

Overall, this ability makes Nasus able to deal a ton of damage while building tank.
This is the first basic ability you'll want to take and max out first.
Higher the ability level, the more damage it deals (however, does not change the damage from the stacks), so it becomes easier to last hit with Q.
But more or so for the ability cooldown, since Q Siphoning Strike becomes available more often to get as many stacks as possible.

W Wither
Doesn't deal any damage but rather strong single target slow ability.
W Wither is a point-click ability so you just click on the opponent when s/he is in range.

It starts off with 35% slow but it increases over time, max of 95% slow at max ability level.
It also reduces the target's attack speed, which is great if you're going in for full trade.
Also a great ability to setup a gank for jungler or secure kills.
During a teamfight, it's best used on your opponent's ADC to catch the squishy out (and reduce DPS).

If you are confident that it'll lead to a kill, you can Flash -> W Wither -> whatever you gotta do.
If you need to escape, you can W Wither the opponent and Flash if necessary.
But oftentimes, W Wither can be enough to escape.

This would be the last ability you'll want to max.
If your jungler is ganking you when you're level 2, then this would be a great ability to take at level 2.
Otherwise, you can get this ability at level 3.

E Spirit Fire
This ability has rather large area of effect.
Initially deals a burst of damage in the area (albeit not that big of a damage), then damage over time for all the opposing units (champions, minions, jungle monsters).
Also reduces the armor of opponents who stay in the area, which is great if you're committing to a fight.
This ability has rather large mana cost, so it's not something you'll want to spam.
It also affects the minion wave too, so if you don't want to push the lane, that's something you want to take into consideration.
Overall, this ability can be used to poke (but mind the mana cost), big trade, and pushing lane.
Typically the second basic ability to max (after R and Q).

R Fury of the Sands
This ultimate ability has a few components that need to be broken down:
1) Fury of the Sands is a self-cast ability that empowers Nasus- press R and it's activated. All the effect of this ult lasts 15 seconds.

2) Deals magic damage to all opposing units around Nasus based on the max health of the target (champions, minions, monsters) as damage-over-time. This scales off of AP which Nasus pretty much has none. Not that big of a damage but some tank shred there.

3) While ultimate is active, Nasus gains extra health, armor, magic resistance, basic attack range, and his size.
He is already tanky- but this ultimate makes him even more tanky.
Increased basic attack range increases his range for Q Siphoning Strike, which gives him better chance to reach the squshies in the backline.
Increased champion size leads to better peeling for backline squishies.

4) On top of #3, Nasus gains additional armor and magic resistance each second, for 15 seconds while the ult is active. Hence, it's better to activate this ult in the beginning of the teamfight rather than when you're about to die.

5) While Fury of the Sands is active, Q Siphoning Strike has additional 50% cooldown reduction- you can pretty much spam Q while ult is active.
This is not only good during the fight (to deal more damage), but it's good for taking objectives or to simply harvest Q stacks.
So don't be afraid to use Fury of the Sands for non-fight purpose.

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