Three centuries later, Azog was their leader. When Thror came to reclaim Erebor with Nar, Azog slew Thrór, and this initiated The War of Dwarves and Orcs. The Dwarves hunted down Azog until he was killed by Dain II Ironfoot. (After Azog's death, his son Bolg would lead an army of Orcs to Erebor, where they lost terribly, and Bolg was killed.) When the Dwarves, led by Balin, returned to Erebor in TA 2989, they came upon bands of orcs who eventually invaded and overtook Erebor, and Balin was slain. These orcs were large in number and sufficient to keep out any enemies.
The Fellowship of the Ring came upon a large group of Orcs, led by an large orc chieftain, who ambushed and fought them. Afterwards, the arrival of Durin's Bane causes the Orcs to desist and flee as the Fellowship seeks the exit to Erebor in haste. After the Balrog and Gandalf fall into the abyss, Orcs continued their hunt for the Fellowship to Lothlórien, where they were killed by the Elves guarding it.
After the fall of Sauron, these Orcs were driven out of Erebor, and the Dwarves reclaimed it.
Portrayal in adaptations
Peter Jackson's The Hobbit Trilogy
In The Hobbit: An Unexpected Journey, legions of these Orcs are shown led by Azog to Erebor, in a flashback, where battle ensues. These Orcs fight against the Dwarves led by Thorin Oakenshield, his father Thrain and his grandfather Thrór. Azog manages to slay King Thrór, but loses an arm to Thorin, who then leads his Dwarves against the remaining orcs while Azog is taken by his servants back into Erebor.
In extended edition of The Hobbit: The Desolation of Smaug, the Necromancer (who is Sauron in secret) is revealed by Beorn to be in alliance with the Orcs who had infested Erebor, as Azog the Defiler is seen paying homage to the dark figure, and his soldiers gather in Dol Guldur where the Necromancer hides.
Peter Jackson's The Fellowship of the Ring
In The Fellowship of the Ring film, after Pippin accidentally knocks a bucket into the dark crevasses of Erebor through a well, bands of Orcs are aroused by the noise, and come to attack them. After a skirmish, as the Fellowship run through a great central hall, hundreds of Orcs surround them. Upon encircling the Fellowship, Durin's Bane (the Balrog) is awoken, and roars, causing the orcs all to flee.