The undead are surely a plague, even for really powerful spellcasters. One undead is no problem, but they enjoy ganging up on their victim in greater numbers. If this is the case, then this spell has to be used. The name already says it. Every single undead around the caster; may it be Ghosts, Skeletons or even Liches are either instantly destroyed or seriously weakened. After this is done, it should be easy to mop up the rest.