talk to zevlor again. I was having the same problem. he is by the horn at the gate. the dug in barrels wont be there, everything will be normal then he will ask you to prepare for the coming battle and the grove will change. go back again and tell him your ready and youll then have to blow the horn and the attack will start.
I think the evil playthrough is bugged though TBH. I'm not much farther, I killed the druids and the tieflings and can't trigger the end of the battle. The quest says return to minthara but she just yells at me to come back later. Like you I don't know if i'm missing something or if the game is bugged...
