Edit: 1-80 physical damage.
Imagine grabbing both PD (or one of the energized items) and Hurricane to go with your LW item and another big ad item. You'll stack the keystone mastery quick and deal an additional 80 damage late game as if you bought an item for it.
And of course I left out [boots] and that sixth item, but those could be anything. Warlord's Bloodlust is the hype right now but I feel that FoB may be a stronger equip.
Fervor of Battle is amazing on Devourer junglers.
I've just been picking up Devourer before building a couple tank items. I don't really feel like I've lost much damage from not purchasing BoRK, especially after I get another on-hit item late-game.
It adds more damage than any core AD item now..
does FoB increase auto damage
or is it a "on hit" that doesnt benefit from crits and lifesteal?
i think its probably on-hit . since it only works vs champions
I'm thinking anyone who typically rushes low ad items like Bork (zed, vayne), triforce (ez, corki) or another attack speed item (kog, yasuo) would be a monster with this. Good thing Runaan's doesn't give 70% attack speed any more
Does anyone know if this applies to Warwick ultimate?
I like to play Fighter 
sometimes with Fervor as keystone.
People never expect the dmg.
Back when they thing came out, you could stack it simply by tossing boomerang into thin air. So stacking it up was so easy.
