The Source taps into the Hollow, (which we only see one other time in the series' end) which he absorbs and puts into demons to steal Piper and Paige's powers. A darklighter shoots Leo, so they have no in-house healer as Paige said. So with the source having two of three charmed powers...he comes to attack. The sisters use the power of three to say the summon their powers using the call a lost witch/ powers spell..but the source is summoned instead. he blasts piper into a wall using a fireball. they meet in the attic and cole saves phoebe from a fireball and he has the hollow in him so he gets the sources powers.. and the sisters use the powers of their ancestors to vanquish the source.
excited episode..the beginning of season 4 part 2