To open the Portal is the main objectives of that mission and to unlock it, players need to attune all three nearby thorn pedestals at the same time using Rune Crystals, as explained in the mission description. This will attune the Portal itself to them and they can pass through it. The other side of the Portal is guarded by the Henge Guardian, a gigantic level 20 Behemoth surrounded by a few Wind Riders and Root Behemoths.