While I am a fan of Tiamat you misunderstand their effect. The splash damage does apply to Nasus' Siphoning Strike but it does not grant him more than one stack if he kills more than one minion with it. If you kill 8 minions with one Siphon Strike and Tiamat, you're still only getting 3 bonus damage, just the same as if you'd killed just one minion with it.

Because of this, I disagree with your inclusion of it in your build. It gives you good damage, HP regen, and mana regen, but all the splash is helping you with is farming and perhaps damage in teamfights, not last hitting with your Q. Nasus is already a very strong farmer with Soul Fire so Tiamat is fairly redundant and there are better items for damage, mana, and HP regen (An Eleisa's Miracle for tenacity for example or Atma's Impaler to take full advantage of Warmog's).

I will be sure to fill in those points.. also some of the things that were left a little blank are because 1. I've already taken an hour to format and write this much and i had to go to work and 2. I'm still experimenting with the build and trying to find the flaws in it myself. your advice is greatly appreciated as it will help me to make this guide better.

A few points of contention with this build.

First, your choice of summoner spells. I honestly can't agree with taking Revive. This spell is only useful if you die, and I'd think it more beneficial if you instead invested in spells that help prevent you from dying. I can see how Revive might help you get back to your lane faster if you're successfully ganked, but Teleport achieves the same effect. On top of that it can also be useful for splitpushing and porting into teamfights. It also has a reasonable cooldown, unlike Revive (eight and a half minutes even with the mastery point).

I also take issue with your Runes. Besides the Seals, I think your choices are slightly misplaced. With no Magic Resist in your item build AT ALL it would be advisable to take either straight or per level Magic Resist Glyphs in order to make laning against AP easier and to give you some extra survivability mid/late-game. Your Marks and Quints are fitting with your stat choice of Crit Chance, but with no Armour Penetration except for the 6 in your Offensive Masteries you are not only giving up a lot of damage in teamfights, it makes your last hitting (very important for Nasus) much more difficult.

Your item build seems fairly straightforward for AD Nasus. However, not having any Tenacity makes me question Boots of Swiftness. Being able to chase with ~450 movespeed and Wither is amazing, but against a team with CC you may find Mercury Treads a preferable option for both chasing and escaping. The boot alternatives listed further down the page are good, but if you recommend Boots of Swiftness I'd also recommend putting in a note about grabbing a Cloak and Dagger for Tenacity (as it synergizes well with an already crit-focused build).

The Tiamat is an amazing addition to this build, one I had considered but did not truly begin to appreciate until you pointed out all of its benefits. The Trinity Force is of course a staple for Nasus - far too good to pass up. He uses ALL of its stats well. The Bloodthirster is amazing. The damage is good, and the lifesteal stacks nicely with his passive. The Infinity Edge and Warmog's are questionable though. If you take a Warmog's, it is expected that you also take an Atma's Impaler. This provides some armour and crit as well as gives you some extra AD from the health Warmog's provides. Alternatively, if you take the Infinity Edge it might be more beneficial to forego Warmog's for the straight mitigation from Guardian Angel.

Finally, your ability sequence is good (maxing SS first and then Wither is always the best choice) except for your choice at level 4. Surely, Spirit Fire is a mana-hog at that point in the game, but I think it outweighs the second rank in Wither. If you get pushed back to your turret with many enemy minions headed at you or are about to participate in a gank, the extra damage and armour reduction makes a far greater difference than the extra 3% movement speed reduction that Wither would give you.

Overall, a very well laid out and detailed guide with good ideas, but with a few rather fatal problems.