One of the parties they sent out into the world had brought back something of deep interest to the Grey-Seer. They had pilfered one of the tomes of Nagash the great necromancer lich. Dumping it on their desk, the party was shooed out the Grey-Seer's quarters. Wanting to dive immediately into such knowledge.

After deciphering the first few chapters they prepare the room for the rite, to their best understanding it was for immortality and great power. Something all Skaven would wage wars for. Wasting no time. eager for the rite to begin they rush the process. In true Skaven fashion the rite becomes unbound, the copious amounts of warpstone in their room had created a resonant-cascade of chaotic energies. The Clan's warren was engulfed in foul green fire. Structures and inhabitants alike were swallowed in its destruction.

...

It had been weeks before there was movement in the warren again, animals and pillagers had seen the explosion and cleaned the dead warren out. But in the darkness a fetid energy now burned from the blasted corpse of the Grey-Seer. Their Skeletal form stirred and pulled itself up.

They had cheated death, though a new hunger had found themselves, to satiate it they would need an army...
Arms stretched out wide they commanded their dead clan-mates to reclaim their vesles, as if pulling them out of their torpor they raised their remains to their feet.
That night blood-curdling screams and manic laughter could be heard all the way to the nearest human settlement.

...

It didn't take long for their hunger to take them out their nest. Their army shambled towards the humans. They could see their souls like beacons to a buffet drawing them to the town.

The Grey-Seer would dine well tonight, their souls filling the rat's "gut"

Something I cooked up for spoopy-month