Flemeth's Real Grimoire

Morrigan will give the player a personal quest which will involve you obtaining her mother's real Grimore after you give her the Black Grimoire.

The ending of the quest
Should you decide to pursue Morrigan's request, ensure your party is prepared for a tough battle. Morrigan says that the only way to get her mother's Grimoire is to kill her and her mother is a shapeshifter.

Once you approach Flemeth and make it clear that you intend to fight her, she will shapeshift into a large dragon which you must defeat in order to enter the house and get the Grimoire. Morrigan will refuse to be present during the quest as she fears her mother will overtake her if she should return to the house.

Outside of the Grimoire, there is not much of note in Flemeth's house, so retrieve the Grimoire and give it to Morrigan to end the quest.

There is an alternative way to complete this quest, although it will net you some disapproval (-5) from Wynne and (-2) from Leilianna, and/or approval from Alistair and Zhevran (+2), if either in your party. Instead of choosing to fight Flemeth, you may, through the correct dialogue choices (begin by asking her whether she really is planning on taking over Morrigan), get Flemeth to give you the Grimoire, as long as you tell Morrigan that you have killed Flemeth. At the end of the dialogue, Flemeth will tell you that you and her will never meet again, and leaves, leaving you free to enter her hut and take her Grimoire and a powerful enchanter's robe (Robe of Possession) to be used for Morrigan. The fact that the robe has a -1 to Willpower and the description associated with it seems to support Morrigan's fear of being subsumed by Flemeth.