Box of Lobsters! Or a musebox.

It had to be negative emotions - pain, suffering, despair. They were much easier to stimulate en masse than positive emotions, after all, and while Mithos was patient, he still preferred that his goals were completed within a reasonable timeframe.

So he could easily reassure Genis, "We'll find the best way to revive her as quickly as possible, and you can still see her almost anytime. We'll be keeping the hosts under observation, so I'll show you where later."

Hosts weren't normally kept under such close scrutiny, but this was different. This wasn't just the cultivation of any Exsphere to add to their stock.