With Winter Campaign you can an immediate attack on target opponents zone.

http://deckbox.org/whi/Winter%20Campaign

1. Is it the units in the battlefield and legend who gets to attack?

2. From wich zone do you attack?

Is it like you get an extra battlefield phase?

Sorry found the answer in the FAQ on side 13 = follow the steps of à normal battlefield phase

Have to ask the question anyway.

1 and 2 above

1) Yes

2) You declare which zone you attack as part of a battlefield phase, so your choice.