This is definitely the most fun i've ever had playing this game. Its a wave after wave survival mode, and from what I can tell by the leader board there are 19 levels, or 20 and no one has finished 20... Anyway, your able to pick 3 different characters and level each one individually. You get new gear and weapons, and there are no other units. Just you, and 2 other players against increasingly difficult opponents. It gets really hard at wave 9 where there are just hordes and hordes of Eldar with platforms and the amount of spinner fire is insane.
The furthest I got was with my first game. We made it to level 9 with 1 level 6 player, and two of us at level 1, all playing as space marine sergeants. My second game we only made it to level 8 with 2 level 2 SM players and a level 4 mekboy who was horrible and constantly died, I think he was downed on every wave.
You can revive downed opponents and you actually have 2 control points you need to hold, and if you claim one during an active wave you get a multiplier which really helps. Points are awarded for kills and the amount of points depends on the difficulty of the unit destroyed.
And one last really fun thing that everyone should be excited about... There is a wave of Imperial Guardsmen. Win.
If you have this game, go play this survival mode NOW. And if you don't have it yet, this is reason enough to buy the game, especially with it being only $20 this week. It is by far more fun than anything else in this game and I think we're going to see a lot more of this gametype coming from Relic.