Seeing a completed dimensional portal wasn't exactly rare. In fact, it had almost become expected in certain places.
If a mid-tier portal was discovered with beasts the guild could handle, most of the time they would purposely avoid defeating the dimension boss. That way, they could continue to reap rewards from the beast crystals inside.
However, if the beasts inside were weak and the guild found the area too easy, they would rather take down the boss, collect its rare crystal, and attempt to open a new portal somewhere else.
It was a common pattern followed by many guilds.
That said, opening portals wasn't something done on a whim. Each time one appeared, someone would record the location and the specific magic incantation used to open it.
That information could then be sold, licensed, essentially, to other guilds, allowing them to recreate the portal and explore the same dimension. It was a highly profitable practice.
