ToME: the Tales of Maj'Eyal

After completing the lichform ritual, I was expecting Celia's heart to be consumed in the process, but nope, it stayed in my char's inventory. And apparently, it can be activated for something... but what? Anything useful?

Using it gets you a soul. It can be used repeatedly, but the cooldown keeps getting longer, so eventually it takes hundreds of turns of waiting to refresh.