That's strong magic. It's made him gravely ill.
6 seconds sound clip from the Spirited Away movie soundboard.
You can hear this line at 01:28:13 in the Blu-ray version of the movie.
Quote context
[...]
- That's as powerful as it gets.
- Zaneba put a curse on the seal.
- That's strong magic. It's made him gravely ill.
- That should make him more comfortable.
- Poor Haku. He's had it hard ever since he got here.
- He just showed up out of nowhere, just like you did.
- But he got mixed up with Yubaba. He took a job as her apprentice.
[...]