Do they work together?

Could you add +8 to other player by Beorn 3+5?

I'm pretty sure this has been asked and answered before - and the answer is no, because Grave Cairn takes effect after the character leaves play, and after Beorn has left play his attack goes back to the printed 3 (though doing it just for +3 attack is still pretty good).

Also he leaves play at the end of the phase so if you trigger his ability during the combat phase to attack with Beorn he doesn't leave play till the end of the combat phase at which point its more or less meaningless boosting another characters attack.

You could theoretically use his ability before the combat phase. It would be weird, but maybe you really want to trigger this combo and give Boromir +8 attack to kill a bunch of enemies.

All I can find is a thread on BGG. There is no official comment, but some people are convinced it's +3 instead of +8. I'm not totally convinced by this argument, but have no official reference.

For example: with Born Aloft.

You trigger Beorn, and he returns to hand by Born Aloft. Beorn has 8 attack even if he is not in table game.

http://www.cardgamedb.com/forums/index.php?/topic/1702-official-nate-rule-clarifications/?p=130504

Q：

After Gimli been destoryed, I play Grave Cairn to add his ATK to another hero, how many ATK can that hero get?(Gimli has no attachment)
A：

When you use Grave Cairn, you are targeting a character who has just left play. When a character leaves play, all tokens on that character are returned to the token bank and all attachments on that character are placed in the discard pile. Furthermore, the game text on dead heroes and allies in the discard pile is not active, so when you play Grave Cairn, the only attack value that you add is the target character’s printed attack strength.

ok thanks