Codex: Orks

Orks are a violent, humanoid race from the Warhammer 40,000 universe that enjoys making war more than anything else. For this reason, Orks can be one of the most enjoyable of races to play.
Their large size and muscular build make them adept at war, and the fact they are the most numerous race enables them to create large armies capable of taking on even elite opposing armies. In addition, Orks are able to customize lost body parts. If an Ork loses a hand or an arm, the stump is covered with a weapon, often making him a better warrior. Even their heads can be customized with weapon accessories. Many Ork kits come with a variety of plastic parts that can be interchanged easily.
They use relatively crude weapons, including stickbombz, large guns called sluggas, brutal edged weapons called choppas and the new shokk attack gun. As a result, Ork tactics usually include massing large numbers against enemies and the use of a Furious Charge. However, Ork armies can also be built for speed, using Trukks, Warbuggies and Warbikes. Regardless, Orks are better in close combat and closing in on their enemy is a favorite tactic as they gain a high number of attacks during melee combat. The only race that can compete in melee combat with Orks are Eldar Aspect warriors. Ork Tankbusta boys are effective against enemy armor, especially Imperial Guard since they tend to use a lot of armored tanks.
As for enemies, nearly every other race in the Warhammer universe can be included on this list, including themselves as they frequently battle each other for various reasons.
The unconventional nature of Orks can make them very fun to play and keeps game play fresh.

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Games Workshop Space Orks Codex