I guess he used the 2nd thorn for the teleportation and will break the spell on the wall by shooting the arrow. But it is still early so let's wait and See.
So far I sticked to the theory where Baam would transfer the exploding battleship behind the second wall, killing loads of people. But now I can see this happening "The biggest twist would be Baam saving Kallavans forces". I would still prefer something similar to the former though. I want to...