It’s from Queen Eclipsa’s chapter, so we know she was the one to master the spell and write it in the book after she did, but what/who did she spy on with this spell? Those monsters in the spell picture? The one enshrouded in shadows looks important.
Toffee did say it was “the most efficient way to spy on your enemies” Toffee’s was electronic though, but he either remembers since he’s immortal and we don’t know how long he’s been fighting or how many Queens he’s had to fight or it’s information he learned from the Mewni royal family history book he was reading up on and made one. Who knows?