So after reaching the Last Light Inn, I went upstairs to talk to Isobel the cleric. After we were ambushed by Marcus, I used Banish on him. I had him completely removed from the fight, went invisible on my wizard and hid in the corner.
After about 2 rounds, one of the winged horrors paralysed her. While she was paralysed, Marcus, still banished, walked up to her, got 3 attacks, knocked her out and took off with her. Banishment had not been broken, my wizard did not get hit. Marcus just got to take a turn while banished.
I check afterwards to see if banishment had a turn limit, and it does not. I don't suppose anyone can shed some light on this?
