Druid Circles are Overworld structures that have a chance to appear in any Overworld Swampland-type biome. They are vital to getting to the Betweenlands.
The Druid Circle appears as a ring of pillars surrounding a mysterious altar. A Dark Druid Spawner spawns underneath this altar. You will have to fight the Dark Druids that spawn from it and obtain four different Swamp Talisman pieces. Then, you can right-click the altar and put them inside; the altar will then combine the pieces into the Swamp Talisman, the item that creates the Portal Tree.
The frequency at which Dark Druid Altars spawn can be edited in the config.