Total Price: 3000
Recipe Price: 1188
Grants: 15. Armor Penetration, 50 Attack Damage, 10% Cooldown Reduction, 250 Health.
Passive: Dealing physical damage to an enemy champion reduces their armor by 7.5% for 4 seconds. This effect stacks up to 4 times.

The Black Cleaver is an item for ranged and melee physical damage dealers. It was greatly changed for Season 3: First of all, its passive is not unique anymore, that means you can stack up to six Black Cleavers and deal " Tons of damage". :D
A very big streamer is complainig about this item in his stream and is using it quite often. I will quote him now, because he has inspired me to create this special guide for you: "If you don't deal enough damage buy Black Cleaver, if you need even more damage buy even more Black Cleavers." (Guardsman Bob)
Blac Cleaver is a very strong item, but it truely shines when you either stack it or some teammates buy it so you have more space for other items, but this is not neccessary,because the Black Cleaver is not just giving yuo damage and CDR, it is also granting you 250 Health each.
So one can say that the Black Cleavers is especially made for bruisers like Pantheon, Rivena nd Fiora. Beacause the passive of the Black Cleaver does not only work for auto attacks, champions like Corki or Graves, who deal physical Area of Effect damage, synegize with it too. They can reduce the whole enemy team's armor with their skills.
That is why Black Cleaver is so strong and so often used since Season 3, it is very good for the laning phase and it is good later on in the large team fights.

Ashe, Caitlyn, Corki, Darius, Draven, Ezreal, Fiora, Garen, Graves, Hecarim, Jayce, Kha'Zix, Lee Sin, Nocturne, Olaf, Pantheon, Renekton, Rengar, Riven, Talon, Twitch, Urgot, Varus, Wukong, Xin'Zhao, Zed

If you choose to stack Black Cleaver, always stack at least 3, so you have got space for your boots of choice, a GA, and another item you like to play on the single champions.
Do not stack 6 Black Cleavers, because you wont be able too chase enemies down, who run with low life. Always try to take a maximum of 4 Cleavers, so you can buy boots and a defensive item like Guardian Angel.
I prefer buying Boots, 3x Black Cleavers, Guardian Angel and a Last Whisper, to maximize the damage output. With Last Whisper and some Blac Cleavers you can shred the enemy's tank in no time and deal tons of damage. :D

I hope this guide helps you, deciding wheather to buy Black Cleavers or not. If you play one of the champions from my Black Cleaver pick list always buy at least 1, just for the laughs and kills. :P