You gradually gain Magic resource. Every now and then its consumed to generate some special event-only resource. To find the resource, you'll need to mouse-over the icon. In terms of where it shows up it can be seemingly in nearly any other place. a normal-icon will swap to event icon. For example Copper in mining, or a Gem in gems. Just go through your various tabs until finding it. It can even spawn on prestige sub-section apparently.
Icons it can be are something like "maple leaf", "grass", "skull", "clover", or a potion bottle thing.
Once you have some, you can attempt a ritual. These gives you points for event shop, can unlock potions and unlock a new resource for the event which so far I've only seen as able to combine multiple resources into potion, and higher chance to succeed.