The Necrons are a faction that was introduced in Winter Assault. Since they make their appearance only in the campaign as an enemy of the player, they are not a playable faction.
The Warriors of the Necron Army are the strongest unit in the game armor and weapon wise. Each Warrior has a gauss cannon, which can kill an entire squad of guardsmen in just a few shots. The Necrons use this same weapon to smash enemies in close combat, generally launching the enemy back or knocking them down. Once killed, every 1 of 8 Necron Warriors will stand back up and fight, but with only half health. Warriors armor is very heavy, able to take quite a lot of bolter rounds before finally dying. Warriors, while seeming over powered, are only good against infantry and light vehicles unless they are in massive numbers.
The Monolith is a massive mobile fortress which has one tri-linked gauss cannon on each corner and a portal of despair which can engulf units in the Portal of Despair located on the front (Tabletop version of the game only.) The Monolith also has a massive crystal located on the top which fires a devastating gauss beam, capable of causing massive amounts of destruction wherever it fires. The Monolith can teleport around the battlefield unless 25% or more of the Monolith is damaged. The Monolith is heavily armored, capable of taking on massive amounts of punishment.