As written Frostbite does almost nothing.
At the end of the round you have whatever cards you haven't played. So in theory if you have multiple non-Quick actions and can't play one of them you're forced to discard (and possibly return) that.
In 2nd Ed there's a cleanup step, so at the end of the round there are no cards at all.
However, the intention for the card was different. It was supposed to be instant.
Append: If you do not Block or Avoid this card, this effect takes place immediately when it is revealed.
Thanks Jeremy -- that's exactly how I played it.
While I have your ear, one more quick question: Some Onslaught cards have an ongoing Persistent effect until you fend them off. Does that ongoing effect take place immediately (if you don't Avoid or Block), or does it only start to take effect at the end of the round (because it's Onslaught?)
I've been having a blast with the expansion, btw. At first I thought it was too difficult, but you do get better with practice, and games are generally close. The final battle really adds a satisfying drama to the endgame.