One of the only things that I remember they appeared randomly across cities.
Some tiny nearly noticeable, other massive rifts that tucked whole buildings or streets.
Each portal has a scenario with a set of rules from the system.. Some can be dangerous, others bizarrely weird.
Society went into a lockdown.
People formed alliances to fight off the monsters, and others exploited many of their deaths to get the rewards.
It was in pure chaos till the awakened appearedred, those are the ones used their powers for good or evil.