There are six purple portal locations. Once entering, you will be transported to an alternate map with floating rocks. You must jump on the rocks to get to the tower at the end of the map.
Four legendaries have the chance to spawn on the tower. Arceus, which must be obtained by climbing stairs spawned at the top of the tower. Dialga and Palkia, which must be obtained by entering a portal of their respective color at the top of the tower. Giratina, which spawns inside his own realm accessed at the top of the tower. Giratina's realm is difficult to navigate, so Giratina is widely considered the hardest legendary to obtain. To make getting Giratina easier, choosing a pokemon with Teleport or Fly such as Xatu, Deoxys, Ralts and its evolutions or Salamence and Rayquaza be a wise choice. All of these legendaries can be summoned by a level 100 Hoopa as well as being randomly summoned by the game.
One portal will always be active in one of the six locations. The first location is to the right, inside the sewer. The second location is inside a cave next to the sewer. The third location is on a hill near the acid area. The fourth location is on a shrine on top of the mountains surrounding the Rotom Town. The fifth location is on top of the handle of the axe in the forest. The sixth location is behind the crystal cave mountain on top of two hills connected by a bridge.