one you have reached the top of the tree try different combinations to try to fall in the middle hole. After you defeat the trent head get hermes water and use it on the old tree.

You go to the top of the Tret tree and fall through the hole in the middle and you will fall into his roots and you will see his face on the wall you will need to it and talk then beat it then he will send you out and he will try and turn every one back to people but his powers are weak so he will send you out of the forest you will have to go to the town by the Mercury light house go to were Mia healed the old man there will be a chest in his house get the emty vial out an go to the fountain and fill the vial with the water and go back to the Tret tree and give it to him and he will turn the humans back.