@K-Lye The part of my brain that tries to rationalise plot-holes, no matter how bad, wants to say that when the MC [absorbed] her power that interrupted, however briefly, the spell that had been keeping her alive for these past four centuries.
But the bitter old man part of my brain wants to say that it's because she just got tired of listening to all these brats, showing up at her door unannounced, breaking her stuff, and getting on her last nerve. It's like in
Return of the Jedi when Yoda, who had been rocking along just fine for 900 years, gets so sick of Luke's incessant whining that he just dies and fades away.