I managed to get the third ending
Spoilers:
To get it you need to touch the wall opposite of the big demon door down the stairs. There is a big room with the lion, bird and snake symbols. You can walk through the giant lion head in the centre to find a purple flask. Once you kill the queen, behind the throne there is an angel statue with a fountain, if you click it you pour the purple potion into it and throw the sword in it. Then you leave as you normally would and get the ending.