Collect your Mother's Soul is a sub-quest that forms part of the quest The Souls of Heroes in Fable: The Lost Chapters and Fable Anniversary.
Return to Oakvale and the Memorial Garden. There your mother's ghost will tell you she is being tormented by Screamers. Kill them and gain your mother's soul.
A way to get infinite experience is that you can stock up with Health Potions, and stay here, killing Screamers and then letting the soul meter go down. They infinitely respawn, until you complete the quest. Bring Ages of Might Potions, Ages of Skill Potions, and Ages of Will Potions to use at a high combat multiplier. In Fable Anniversary, this will unlock the Combat, Evolved achievement.
If you choose to do this instead of Killing Briar Rose, you are likely to gain more experience.