Not Updated For Current Season
Ghost + Exhaust = My favorite combo, because increased speed and lowered speed are always nice. Exhaust is always nice when that carry who is annoying can't attack.
Worthy Mention: Fortify - People can use his ult to pull people into their tower, slow them, and then use fortify to deal a boost in damage. I very good idea, and coupled with exhuast can work even better
Flash: I understand why, and the flash + terror capicator slow can be very useful.
Teleport: Nice, but not my cup of tea.
Clarity: Also see why, but I find urgot going out of mana early game, and I rememdy this with a chalice, so it's not good for this build.
Ignite: For those run away guys, also understand, I personally don't like.
Cleanse: Maybe, if you like using your ult alot.
Clairvoyance: Whatever, if you want it, get it.
Heal: Somewhat understandable with the ult, but I don't like.
Rally: No need.
Revive: You shouldn't have this, just makes you look like a noob.
21/0/9 is always good, but i just have a few notes about it. For the offense tree, you can customize anyway u want, if you want more damage capability get deadliness and alacrity and other physical DPS stuff, and vice versa for AP.
Yellow: Attack Speed - Attack speed is very important because his passive is just a great thing, and his damage is pretty high, and we only get a small, modest boost to his normal damage, and he does wonder.
Red: Attack Damage at 18 - We want damage for acid hunter, plain and simple. You can change this for base damage, or for crit chance, if thats what you want.
Blue: AP - This is hybrid. We want AP + AD. So, lets just use the best AP runes around!
Quints: AS, AD, and AS - Just a combination of the three above.
Early Game - Be careful with your acid hunters until you get chalice, and even then, be careful. Once you get to about 4-5, you can start racking up the damage with charge + acid. When you get your ult, and if your with somebody, then cordinate so that he launch a slow/snare/cc. Only use terror capicator when you need the slow, and when u need a little boost to lower the damage of a nuke.
Mid Game - I'm not going to say you should be racking up the kills, because some games just don't go that way. Right now you should have your guinsoo's, or are close to it, and are having fun with your charge + acid hunter attacks, and might even have a kill or 4. Here you will probably cordinate ganks and your teleporter helps alot. Pick a guy, swap with him, and take him out. Mana here becomes less of an issue, but can still limit you.-+
Late Game - You probably have nashor's tooth and you have sheen. At this time, you should be doing alot of damage, and people fear your missles, and others just love getting hit by it. Always try to hit people with charges, and also, charges work wonders with minions, and allow you to farm very well. And when you swap, do it quick and when its done RUN THE HELL OUT OF THERE! Your team is gonna know when you swap, and if the other team is good, they'll know too. So the second they swap, put up terror, and RUN!!! Mercury treads help at this time, but you have already tanky stats, so you will probably survive unless they give you alot of CC.