The games themselves are unspectacular but generally pretty decent strategy/RPG affairs, but for the set price of £3.89 you can't really go wrong. You've got two weeks to make use of the bundle deal.
As for that DLC, you'll get; King Arthur II: Dead Legions, King Arthur: Fallen Champions, King Arthur: Knights and Vassals DLC, King Arthur: Legendary Artifacts DLC, King Arthur: The Druids, and King Arthur: The Saxons. Most of these add new units, monsters and campaigns. Not all of the extra content is great, but there's enough there to keep you busy over the holidays.
In case the series has passed you by, King Arthur has you play the(not-very-memorably played by Clive Owen in the ) in his effort to conquer the warring provinces of a fantasy-heavy alternative Britannia. Alongside big, Total War-style battles you'll make various moral decisions and personal choices which will affect the development of the campaign. It can be a bit shonky and it looks rather muddy and uninspiring, but there's some decent strategy battles to be had here.
All your bundle keys will unlock on Steam. The King Arthur Complete Bundle is available for two weeks.