The Fel Horde is a huge Ork force that emerged around the year 2171, after the UNSC's bombardment of Orsinium. The most notable of all the orks - their leader - is the cruel and cunning Warlock Gul'dan, who heads the true leadership behind the orkish horde, the insidious Shadow Council.
Ironically, the apparent destruction of Orsinium did little to the orkish people. Now, after the conclusion of the Ork and Sergal War, the greenskins are more aggressive and violent than ever. Their population sprang back up, and now they are more willing to conquer than before.
The orks as a whole were so furious about the destruction of their homeland, that when the conniving but wise Gul'dan appeared on the scene, the orks forgot about their distrust or hostility towards magic-users, and quickly rallied behind the warlock. This was only strengthened when Blackhand the Destroyer, a mighty and fear-inspiring iron-clad ork, was sworn in as the new Warlord of the Fel Horde. During the age of the Ork and Sergal War, the WAAAGH commanded by Ghazhgkull Mag Uruk Thraka was the largest force of orks yet seen on the planet. However, by the time the Fel Horde under Blackhand struck against Velous once more, it was realized that the Fel Horde is indeed far, far larger.
Gul'dan made a pact with Kil'jaeden, demon lord of the Burning Legion, binding the ork people to the demons as they go about their crusade. He rallied thousands upon thousands of ork survivors and formed the Fel Horde, a bloodthirsty, demonic power-imbued, powerful ork force bent on crushing puny humanity and claiming the universe for the orks. Currently, they have re-commissioned several forktresses and have once again begun invading other territories in Velous, where they opened a Dark Portal, through which the demonic Burning Legion has invaded Earth. As we speak, the Fel Horde assists the demonic Legion, serving Gul'dan's will as they rampage across Earth, destroying everything they come across.