As many theories have been placed on what Flemeth is, I feel that there may be a possibility that Flemeth has indeed possessed Morrigan as she fears later on when presented with the Grimoire. The assumption is based directly during Dragon Age: Origins before the Attack on Denerim when Morrigan offers the Dark Ritual. She claims to all of a sudden understand the Dark Ritual and learned it from the Black Grimoire, when the truth may be that Flemeth, or at least a part of her soul, has found its way into Morrigan (quite possibly from the Grimiore or placed on her from since her childhood). Not only does she claim to unravel the secrets at the right time, but she speaks quite directly (as like Flemeth) rather than an a harshly worded unpersuasive direction. The fact the child will be half-archdemon and half-mage would better suit Flemeth rather Morrigan. Also what leads me to this is that Morrigan acts off of whim (reveal Grimoire -> Kill Flemeth) whereas Flemeth seems to prepare as if she knows what will happen. Could Flemeth have taken over Morrigan? Possibly! Would the Dragon Age: Origin Ending still Work? Most Certainly! Does it make Sense? Based on the Codex? Yes! So Morrigan may actually be Flemeth!!