Introduction
Nasus is a seldom seen champion within the League today. His early game is perceived as one of the weakest among top lane champions. This is mostly because, in my opinion, his kit is misunderstood. At first glance, it seems like it's designed around his
Siphoning Strike's damage stacking mechanic, leading to most of the Nasus play that is seen revolving around getting this number as high as possible and itemizing in a way to maximize your number of uses in a teamfight. He's seen as a tank/hyper-carry in this sense, usually building general tankiness and Trinity Force for that on-hit goodness with every cast.
What if I told you there was an easier and faster way that can take his effectiveness to a whole new level? That it is possible for Nasus to be useful at all stages of the game? That you could make it nearly impossible for opponents to itemize against you? That you could live in the enemy team during fights and still build damage items?
Welcome to my Midlane AP Nasus guide.
Summoner Spells
I like to use this spell for two reasons. Nasus has decent sustain if he can get in to hit a target, but if he gets zoned and harassed he has a lot of trouble keeping his health up. This spell will allow you a bit of a buffer on those occasions. Nasus is also a great baiter, able to cast his ultimate and gain an extra 300-600 health depending on his ranks in it. This spell will allow you to survive through more if you decide to bait an aggressive move out of your opponents. It's also still useful in teamfights to give your team a bit of a buffer and allow you to stand in the middle of the fight more.
This is a great all-around spell for anyone. You can use it to initiate or to escape over walls very efficiently. I wouldn't recommend going without it on Nasus, though you might be able to manage if you substitute out some damage for a bit more tankiness in your items.
Other spells to consider
This can be useful if you want to pressure multiple lanes. Nasus'
Spirit Fire is extremely good at pushing waves, especially with AP to back it up. If you think you can do without a bit of lane presence for some mid game pushing power, this can be a good choice.
If the enemy does not have much reliable hard CC, or if you want that extra power to disengage or respond to an attempted disengage from the enemy, it might be useful to run this instead of
Flash. The overall mobility of the spell is superior, but it is spread out over 10 seconds instead of instantaneous. With your massive power in the mid and late game combined with the slows from
Wither and/or
Hextech Gunblade you should be able to keep up with most targets with the help of this spell.
If you're looking for early kill power or your enemies have a champion with high sustain (for instance,
Swain or
Dr. Mundo), this is always a great option.
For when you want to be the king of push. Useful if you prefer to roam, as Nasus can clear waves extremely fast. His
Wither allows for some strong ganks due to its massive slow.
Abilities

Nasus drains his foe's spiritual energy, giving him a natural 14 / 17 / 20 % lifesteal. The lifesteal value increments at levels 1,6, and 11.
Even though you are building AP, this is still a valuable tool in Nasus' arsenal. It is his only innate sustain, and it is really good when combined with a
Sheen,
Lich Bane, or
Trinity Force as the bonus damage will count towards your lifesteal as well.

Nasus's next attack will deal an additional 30 / 50 / 70 / 90 / 110 damage plus the extra permanently gained damage. Siphoning Strike permanently gains 3 damage whenever it kills an enemy champion, minion, or neutral monster.
This is Nasus' main source of damage during the late game, but it requires some TLC to get there. During the laning phase it can give you a little bit of extra damage, but it's very tough to get close enough to the more mobile mid lane champions. I suggest putting a rank in it at level 2 and maxing it last, last hitting with it whenever it's off cooldown. When you get into combat situations, remember that it is an autoattack reset, so try to land an initial attack before pressing Q to maximize your damage.

Nasus ages his target, slowing their movement and attack speed by 35% and reducing it further to reach 47 / 59 / 71 / 83 / 95% slow at the end of 5 seconds. If the duration of the slow is reduced, the extra slow will be applied faster for it to reach its maximum at the end of the duration.
I honestly believe this is one of the greatest slows in the game (right next to
Nunu's
Ice Blast). 35% is already a nice slow, but increasing it to 95% over 5 seconds is just ridiculous. This is one of your most valuable teamfighting tools, as you can lock down the enemy AD carry or bruiser with this one spell. By the time the teamfight is over, you can easily catch stragglers with it. Outside of teamfights, it's a great offensive and defensive tool, keeping enemies away from you or allowing you to catch up for a quick
Siphoning Strike. This should get maxed second to get advantage of the full slow in the mid game.

After a brief delay, a target area becomes desecrated for 5 seconds. Enemies caught in the area when the desecration occurs are dealt an initial burst of 55 / 95 / 135 / 175 / 215 (+60% of ability power) magic damage. Additionally, enemies in the area are dealt 11 / 19 / 27 / 35 / 43 (+12% of ability power) magic damage each second and have their armor reduced by 20 / 25 / 30 / 35 / 40 while on there and for 1 extra second.
When you play AP Nasus, this skill is your bread and butter. You use this for everything from pushing the wave to farming to fighting the enemy team. If you place this in the middle of a teamfight and get the full duration its damage from AP is at a 1.2 ratio and, as an added bonus, it reduces enemy armor by a flat amount for up to 6 seconds. You can almost negate the bonus from a
Chain Vest just by placing this down. This is almost always taken at level 1 and will always be maxed first for pushing/farming potential.

Nasus becomes empowered in the sandstorm for 15 seconds, gaining 300 / 450 / 600 health. While the storm rages, he drains 3 / 4 / 5 (+1% of ability power) % of nearby enemies maximum health each second (240 damage max per second per enemy) and converts 6.375% of the damage done into bonus attack damage for himself until the sandstorm wears off.
Believe it or not, this can become a major source of damage in teamfights. If you have 300 AP and are next to an enemy who has around 2500 maximum health you will not only deal 200 damage every second but you will also be gaining about 13 AD for each tick of damage. And that's just if you have one person next to you. The bonus health allows you to survive longer in the middle of the enemy team, which will allow your ult to tick more times and give you massive amounts of AD to take huge advantage of with the rest of your kit. Grab a rank of this whenever possible to maximize its damage and your durability during teamfights.
Items
This is pretty close to the standard starting items of almost every champion right now. Nasus can choose to forgo a third health potion in order to use his
Spirit Fire more often due to his innate sustain with
Soul Eater. If you are less confident in your ability to make use of his lifesteal (such as when you're against a highly mobile champion like
Ahri or
Katarina) you should probably opt for the third health potion.
Chalice of Harmony is possibly the best item for Nasus during the early stages of the game. With the combination of this and your regen from runes and masteries you can stay in lane for as long as you please.
Elixir of Brilliance is actually a great item during the early game, as it gives more AP than an
Amplifying Tome along with some CDR for 4 minutes, enough time to farm up for your next buy, for only 250 gold. It's one of the most cost-effective ways to get the stats you need early on in the match. Grab both of these as soon as you can.
After this point, build according to your matchup. Here's a list of items that are great on AP Nasus and why they're so good for him.
Possibly the best item to improve Nasus' damage output, regardless of what build you're running with. At level 8 (when I usually pick this up) his base AD is 81.3, which will be added again onto any
Siphoning Strike you throw out due to
Sheen's passive. You should always get this item at some point, but if you're having trouble landing
Siphoning Strike on the enemy you can probably delay it for a couple of minutes.
The mage-friendly brother to
Sheen, this is what makes AP Nasus work in the late game. With a full build, you'll probably have at least 250 AP, which means you get at least 250 bonus physical damage added on to your
Siphoning Strike. This will get converted by lifesteal, so with each use of Q late game you should heal around 130 health if you've been farming it correctly.
A great way to make use of your early
Chalice of Harmony once you shift into the mid game. This item gives you even more mana regen along with some AP, which will allow you to farm to your heart's content.
The spell vamp from this item will not work on your
Siphoning Strike, but your other skills will heal you a bit. It won't be a whole lot with
Spirit Fire, but it increases your durability while in your ultimate by converting the % max health damage into healing for you.
If you thought it was hard to get away from Nasus before, this item will make it even tougher. The slow is great to use on targets once your
Wither wears off, and the combination of spell vamp and lifesteal increases your sustain immensely, especially during teamfights.
Due to his preference to be in the middle of the enemy team, Nasus loves this item. Your AoE presence in fights is huge once you grab this item, especially since the health and armor make you a hell of a lot more durable. Sadly, the passive's damage is not affected by spell vamp.
For those times when you aren't durable enough, this item gives you great resistances and a really nice revive. If you find yourself getting bursted down or focused during teamfights, this can be a great solution and will give you a few more seconds to wreak havoc. Keep in mind, though, that
Fury of the Sands deactivates when this item kicks in, so don't expect to have that AoE churning while you wait to revive.
A common trend has been to buy
Kage's Lucky Pick if you're falling behind as an AP champion. Upgrading it to DFG is usually the better choice of what you're given, and this gives Nasus a nice amount of front-end burst to open up fights with. Even though he isn't the burstiest of champions, he can still benefit from instantly chunking away 30% of someone's health.
If you're having trouble keeping your enemies within your "terror zone", as we'll call it, this is a good item. Even though Nasus' damaging abilities will only apply the 15% version of the item's slow, it's still very nice for keeping people within your reach.
The passive from this item synergizes extremely well with AP Nasus' inclination to stand in the middle of everything. Your ult will benefit a lot from the MR shred, giving you more AD during a fight, which means that your autoattacks and
Siphoning Strikes will benefit from this as well.
This is a fairly good mid game item if you're having a bit of trouble sustaining during fights with enemy AP champions. It will increase any healing you get from your passive and any spell vamp you've bought, making it great against sustained damage mages.
The definition of luxury item on AP Nasus. If you've stomped your lane, already have your
Lich Bane, and aren't dying during teamfights you could pick this up to make your
Siphoning Strike even more devastating.
Gameplay
Your main focus as AP Nasus in the mid lane is to farm. Farm farm farm farm farm farm farm. If you're not farming, you're wrong. You need to accelerate yourself into your hypercarry status as fast as possible so you can be a huge threat in teamfights. Whenever it is off cooldown, farm up the bonus damage for your
Siphoning Strike. A good goal is to aim for at least 1/6 of your creep score going towards that bonus damage.
During the early game your
Spirit Fire can be used to farm up creep waves or harass your opponent. Placing the middle of the AoE slightly behind them will ensure the initial burst hits as well as some of the continuous damage. Make sure whenever you get into a fight around creeps that you place this down. Not only will it be damaging the enemy creep wave, but it will reduce your lane opponent's armor to allow your creeps to do more damage. Take both your own and the enemies' wraith camps whenever possible. You can usually take the whole camp in a few short seconds by placing down your
Spirit Fire, autoattacking the blue wraith once or twice, then using your
Siphoning Strike to finish it off. If your jungler is more gank-oriented and can go without it, take wolves as well.
Mid game you should start to come into your own. You have enough AP and base damage to deal decent poke with your
Spirit Fire, enough mana regen to cast it often, and decent physical damage with your armor shred and
Sheen. When contesting jungle buffs or dragon as a team, I would recommend playing the role of zone control. If you farmed well in the early game you should be able to drop
Spirit Fire on top of your enemies easily and, due in most part to your sustain, you're tanky enough to at least hold off the enemy team while yours finishes taking the objective. Your damage potential isn't at its highest just yet, but it is up there so don't be afraid to jump into fights.
Past the 30 minute mark Nasus becomes a beast. With this build your
Spirit Fire and
Fury of the Sands do a lot of AoE damage and your
Lich Bane enhanced attacks can take out chunks of your opponents' health easily. Try not to be directly on the front line as you get ready for fights, opting instead to be just behind your main tank/initiator, as without your abilities active you are relatively squishy. Watch for your team to initiate and jump on the squishiest target with
Spirit Fire/
Siphoning Strike. Try not to activate
Fury of the Sands until you are actually in the middle of the fight in order to maximize the damage from it. Using
Wither on the enemy AD carry is great if they are causing trouble, but if their damage potential is low use it on your team's main target to keep them within your range.