S02E21: Apocalypse Not

When a vanquishing spell goes awry while the Halliwells try to destroy the 4 Horsemen of the Apocalypse, Prue and one of the Horsemen become trapped in a vortex. Piper and Phoebe enlist the help of the three remaining Horsemen to free Prue and the fourth Horseman before the vortex closes, and both are killed.