@sdijkens:
- Put the event id inside the on_startup section in 00_on_actions.txt. The event itself should just be in a txt file in the events directory.
- Yes, that should be possible by setting the "reformed = [reformed religion]" to the same for multiple religions. You would probably have to mod the event that fires on reformation to set the parent-heresy relations and to ensure that all relevant rulers get the option to convert. You might also have to randomly change some of the other heresies' provinces to the reformed religion to prevent the reformed version from instantly becoming a heresy if one of the other heresies had more provinces (as not all provinces of the reforming religion will switch to reformed at once and I don't think that the other faiths' provinces will switch automatically).