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.
Every 100 levels get a 5x dark cookie production boost! This can be very useful for players hit a wall around novemvigintillion cookies!
Levels 101-200 get a bakery boost and dedicated player upgrade!
Levels 201-300 get a Rainbow Storm boost and Enchanted cookies!
Levels 301-??? get a ???