The Chaos gods get bored with the Warhammer Fantasy world and wander off into the Warp to commence with other evil shenanigans and the Warhammer world is found by Sargeras who immediately plans to invade it. The Burning Legion starts the invasion by infiltrating a number of factions with Dreadlords to sow confusion in their enemies. They also commandeer the old portals that Chaos used at the planets poles as gateways for their forces. Sargeras himself is not allowed to enter the battle. Otherwise the Legion is unrestricted in what they can do. If they've used a tactic before they can use it here.