The Mystic Portal is a special item in Cookie Collector 2. You send cookies through it to upgrade it. It upgrades to a new level every 1000 cookies you send, but the amount of cookies that costs 1 cookie gets higher every level.
Every 10 levels, you will unlock a new upgrade. These can be bought with Dark Cookies, and simply boost common things such as loot bags, daily gifts, cupcakes and more.