Hi, i am Mack3031 and this is my first build and i decided it will be Nasus.
i picking Nasus because i haven't seen any high AD builds for him and i said "might as well".
and before anyone asks, no i do not think as myself as a pro or expert.
also i'm not good with runes any and all advice on the rune section will be thanked

i'm not that good with runes yet but, with the build i find that
Greater Mark of Desolation x9 (Armor Penetration)
Greater Seal of Focus x9 (Cooldown Reduction)
Greater Glyph of Strength x9 (Attack Damage)
and
Greater Quintessence of celerity x3 (mana regen per level)
fit the build pretty well, the Armor penetration and Attack Damage works to make sure you do as much damage as possible for each attack, the cooldown reduction fits those who like spamming there Siphoning Strike (Q), and the Mana regen allows you to use your Wither and Spirit fire and later don't have enough mana to do anything else

i would love some other opinions with the runes part of the build other then my own

For this build going with Ghost will help you catch up to an opponent, or simply get away early game.

Ignite is changeable. i grab it because i find to many times that an opponent is to low and flashes out, Ghost is still on cooldown, and i don't have enough mana for a Spirit Fire.

Other Items

Flash - i find that its good for the quick get away or catch up for the melee attack. but Ghost is a is just as good, if not, better

Teleport - good for getting back quickly, but the cooldown is, i find, way to long

pros
High hitter with build in lifesteal.
Siphoning Strike's Passive makes it even stronger
has a destructive ultimate
has is own slow

Cons
mainly single hit
low armor
build costs a lot
low attack speed
Champions with Thornmail hurt a lot

Early Items

during the start of the game getting the boots and a few health pots is vary important for staying up and grinding up on creeps

Core Items

Both the Trinity Force and the Tiamat make this build powerful
using Siphoning Strike counts as activating an ability so Trinity Force starts to give the 150% basic attack boost, Siphoning Strike is a Basic attack boost meaning Tiamat deals its splash damage, and late game it means a lot of creeps were killed by Siphoning Strike gains 3 times the creeps killed as damage.

Late Game Items
there are a lot of different items to pick from that would help the build a lot, but i picked my favorite items.
Infinity Edge
Is my favorite item to get for this build, although its only a 40% chance of getting that Crit, i find it worth the chance. for the reason of that 400 dmg attack can go ALL the way up to a 1200 dmg attack, without using Siphoning Strike

Last Whisper
Champions with high armor can be a little annoying so the extra armor penetration will allow you to do more damage.

ANOTHER TIAMAT
something that most people don't know is that the passive from these STACK, meaning doing this effectively increases your basic attack by 100% hitting nearly enemy's with FULL damage, hell if you really wanted to you could win matches by getting the Berserkers Greaves,
and not 3, not 4, but 5 TIAMATS!!! (250% splash damage) and yes, i have done this

Phantom Dancer
i figured this out during a match when i though that the 30% crit chance would be better then the 40% penetration, this is more of an optional thing but its best if you switch out Last Whisper with the Phantom Dancer

Nasus can be very team work dependent.
what i do during team fights is look for the most threatening (or the one with the most people around him/her) and target it with Wither, and take it out, and when the enemy is running away i use Ghost to catch up to the highest one, use Wither if the cooldown is done, and allow the kill.

Farming at any stage of the game is mostly the same, hit minions until there low, then finish them with Siphoning Strike, also a fun one once you have the Tiamat is lower minions with Spirit Fire, and hit the Melee minions with a Siphoning Strike.

many finish whatever you can with Siphoning Strike to raise its damage fast.

as you can most likely tell, this is NOT, repeat, NOT, a solo, or Ranked build.
this is just a fun build i make one day, and it was fun when its build up.