This is a novelty guide, I think...
just because of how out of the ordinary it is.

Gonna be short and sweet, because Nasus isn't really hard to play at all, he's a cheap "recommended" champion and he's pretty straightforward.

If you want a serious build, go over to my Kog'Maw build.
This one is just something to try.

It's one of those builds that relies on the unexpected
to get you your kills.

Any good Nasus player builds Trinity Force and farms Siphoning Strike throughout the game.

But if you combine that with Wither and force them to trudge over your Spirit Fire, you maximize your damage. If you have Rabadon's Deathcap, that's a hell of a lot of extra AP, which is extra damage for Spirit Fire. If you have Lich Bane, it converts that AP to attack damage, and procs on TOP of your bonus from Trinity Force.

It works, though obviously it isn't his most viable build. Have you ever seen Siphoning Strike throw over 1000 damage before crit is even factored in? Siphoning Strike should have a +150 bonus every 18 minutes or so. Figure an average 5v5 game goes for about 50 minutes. That's 150*50/18=416 damage, it already does 110 damage on it's max level, plus your attack damage of 175, plus 180% of that (315) because of Trinity Force, plus 250 from Rabadon's Deathcap and Lich Bane.

416
+110
+175
+315
+250
______
1266 damage from Siphoning Strike

Plus, you're doing 215 burst damage with Spirit Fire, and say it takes them the entire length of the skill to walk across it because you hit them with Wither. We also have to factor in the AP bonus to Spirit Fire which is going to be 150 on the burst, so a 365 damage burst, and then the damage over time is going to be 43 per second, plus a 12% bonus, so an extra 5 damage. 48 per second. Over 5 seconds. 240 damage.

1266
+ 240
+ 365
______
1871 damage on your burst so far.

Oh. Let's not forget to factor his ult in.

240 damage per second, if your enemy has enough life to max out the skill's potential. Say they don't. Say they only proc about 200 damage per second on it. So you pop your ult and you use that as part of your burst. You won't be able to be on top of them for all 15 seconds. Say 10 seconds are actually effectively used damaging the enemy.

So now we're at 2971 damage on your burst.

Oh wait- your ult also converts the damage you're doing per second into attack damage. You're also going to have your Siphoning Strike cool down at least once before they can get away. So add another 1266 damage. Figure you get 4 auto attacks off during all this (and you'll probably actually get more than that).

You do 175 attack damage, plus your ult, which is an extra 6% of the damage you deal over time. So we dealt a total of 200 per second, that's going to be 1000 over the 10 seconds of effective damage, so 6% of 1000 is 60. Add 60 to every attack. 235 (175+60) four times is 940.

So with this build, you have a potential burst of 5177 damage before mitigation.

Oh, and Ignite does another 410 of true damage.
So 5577 damage on one burst. As far as I know (I'm probably wrong) that's the highest burst in the entire game.

....Supposing the cosmos line up and you get everything landed before they get away. I ran exhaust to make sure that doesn't happen. After wither wears off, pop exhaust. By the time that wears off, wither should be off cooldown.

This will work on any champ that has an activate/strike type ability such as Talon, Gangplank, or Renekton, though it won't be as effective because only on Nasus do you have the farmable Q and the ult and the AOE skill playing into the equation.

I put in:
Chalice of Harmony for the early mana sustain
Mercury's Treads to minimize CC, and also for magic resists
Randuin's Omen for the life, armor, CDR, and active.
Aegis of the Legion for the armor, magic resist, and life

Obviously, you'll sell Chalice of Harmony once you don't need it anymore, to make room for better items.

Nothing in this build leeches because Nasus already passively has life leech.

I chose these runes for sustainability. More magic resist than armor, because most of the damage in the game comes from abilities.

Nothing really special to say here.

Same as above. I chose them for sustainability and to get the most out of my summoner skills.

Exhaust keeps them close to you, and help you escape, it stacks with Wither, and helps you get your burst off more effectively.

Ignite adds some true damage to your burst, and it's great when the enemy just BARELY gets away.

Hope you have fun trying this. It really is a sight to behold.