Mysterious Portal

The Mysterious Portal instantly opens at the start of the Old One's Army event. The Mysterious Portal cannot be destroyed normally, and two of these will spawn 120 tiles away from the Eternia Crystal Stand each, on both sides. They are responsible for all the enemies that spawn in the event, and they despawn upon the event's completion or failure.

Tips

 * A good way to keep the enemies at bay near the Mysterious Portal is by placing an Explosive Trap near the portal's entrance. This way the enemies will instantly take damage when they come out.
 * Beehive/Hive spawned bees could be used to destroy the portals. Destroying both portals prevents enemies from spawning, making the event impossible to complete.
 * One could use this to destroy a single portal, thereby funneling enemies towards one side of the crystal only.