When Sirius returns from beyond the Veil with knowledge of Voldemort's Horcruxes, he takes on the task of finding and destroying them.
This is a great premise for a story! =) B/c 'going at it alone, each person separately, while having no idea what needs to be doing nevermind how to do it in the first place, is a rather *bad* premise. (I am imagining that this fic is as good as I think it will be! *hasn't read yet*)