The mythallar is a device that can be uses it to store magical energy. Most commonly used commonly in magic items that have limited uses, but there's reports of wizards powering up spells but that's ill advised.
It was created by Tobias Sebastian of Vass in 1477 and since it's creation they've proven to become quite efficient with the smaller mythallars being able to be recharged in a matter of hours rather than the days they used to be.