Conversation so far:
How does season 7 of Buffy the Vampire Slayer start for Willow Rosenberg?
The seventh season starts with Willow Rosenberg in England, unnerved by her power, studying with a coven near Giles' home to harness it.
Why is Willow Rosenberg unnerved by her power?
Willow Rosenberg fears returning to Sunnydale and what she is capable of doing if she loses control again, a fear that dogs her the whole season.
Does Willow Rosenberg ever return to her home?
Yes. In the final episode of the series, Chosen, Buffy Summers calls upon Willow Rosenberg to perform the most powerful spell she has ever attempted.
Does Willow Rosenberg use her power during this season?
Yes. With Kennedy nearby, Willow Rosenberg infuses every Potential Slayer in the world with the same powers Buffy Summers and Faith have.
What happens when Willow Rosenberg casts the spell?
The spell momentarily turns Willow Rosenberg's hair white and makes her glow—Kennedy calls her a goddess—and it ensures that Buffy Summers and the Potentials defeat the First Evil.
Does casting the spell harm Willow Rosenberg?
No. Willow Rosenberg is able to escape with Buffy Summers, Xander Harris, Rupert Giles, and Kennedy as Sunnydale is destroyed.
Anything else interest happen during for Willow Rosenburg at the end of seaosn 7 of Buffy the Vampire Slayer?
Potential Slayers from around the globe congregate at Buffy Summers's home and she trains them to battle the First Evil.
Where did they go when Willow Rosenburg and the rest of her group escape to at the end of season 7?
Willow Rosenburg and the rest of her group escaped from the dark caverns to above ground with the rest of civilization.

Question: What did Willow cast the spell to destroy?
Self-contained rewrite: What did Willow Rosenberg cast the spell to destroy?