As the GM you can technically do anything, "Rocks fall, everyone dies" is within your purview. Creating such a thing as a spell is overpowered to the point of ridiculousness, this is literally an end of world event, more suitable for narration than being played out. Perhaps visions of such a thing can be introduced to ratchet up the storyline's tension.
I would make one individual lich out of one rat, and then throw it into a "swarm." Multiply this process as needed.
Use Zombie Horde as an example. Once the horde dies, individual zombies pop out.
First time I GM'd, this is exactly what I used to throw in some flavor. I had a zombie horde walk in, but spells started firing out from the middle of it.
I had spellgorged zombies, and was keeping track of the swarm's spell castings.
Which I made very apparent once the horde split into individual zombies, and then revealed the spell casting zombies.
Sounds like something I might do to one of my clerics who is trying to achieve litchdome. Like a worm that walks with all the powers of a litch too. To destroy it permanently you have to find the Phylactery. Thus, every time they encounter it, it's better prepared.
Green Ronin's Advanced Bestiary has a template that turns any non-swarm base creature into a swarm of tiny versions of itself.