Above there are two different builds that are both very disruptive if you play the builds correctly. The first is meant to be tanky while being able to do major damage but not carry. The exhaust is very good when paired with the wither as mentioned in a later chapter. The second build will be a very strong carry and can counter even a fed Tryndamere with correct game play.
plus the ult both builds will have +3500 health but what makes them different is that the first will get kills for your team and the second will carry your team. So, if your team doesn't look like it has two strong carries, but has a strong tank you should jungle and use the second build. On the other hand if your team does have 2 carries or a jungler already you should use the tankier build and solo top or play with a ad range or melee and help them carry your team by feeding most of the kills around to your carry.

For build one you getso u can double slow every once and a while to help your carry get kills.
The reason you get is so that if you are solo laning you won't lose a tower when you need an item or just if you are back dooring and trying to ride a wave of minions to victory.

For build two you get so you can secure kills with and , a deadly combination. This can also be used to escape if you are on the wrong side of LUX'S LASERBEAM!

Because of Nasus'farming is very important and when you jungle you need to wait for the monsters to have less than 100 health at level 1.
When laning or just creep farming use your the use your to finish them off.