Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
The first items up to Rod of Ages is EXACTLY how I build up to RoA...the rest of the items just build as you progress.
Some people say despair is terrible, but it usually gets me kills when at lower levels and just keeps getting better as you level, make sure you can flash and run along a person with low health while despair is hitting them then just wait for tantrums cd and get some kills :D.
Anyways on to the explanation...
Rod Of Ages
Rush this item, make sure and get the mana ruby + two health pots first.
Get first boots right after you get rod.
Go for Glacial Shroud immediately after.
After you get Glacial Shroud go for Boots of Swiftness, unless there is high amounts of stuns on their team then go for Mercury Treads.
Then go straight for Sunfire Cape, it adds tons of damage.
Then go for Abyssal Scepter which stacks with your passive, which is lulz.
Go for the Rylai's Scepter then the game is over if you get your ult + tantrums + despair + Sunfire Cape + Ignite, you're guaranteed atleast one kill with ignite.
You can also grab Frozen Heart for Capped CD reduction after Rylai's Scepter.
Remember, if you miss a bandage toss you can always flash in and ult!
Have Fun! :D