Invasion was not an unfamiliar concept in the wider universe.
It simply meant one realm taking control of another, or being swallowed by a stronger one.
There were many reasons it happened.
Some realms were rich in resources that could not be replicated elsewhere. When such resources existed in limited quantity, conflict was inevitable.
Other times, it was about position.
Certain realms sat at key junctions between planes, acting as natural gateways for trade, travel, or military movement. Controlling such a realm meant a lot of things. In a universe where distance could be crossed in moments, location still mattered.
There were also realms that were simply weak.
Newly formed worlds. Damaged realms recovering from catastrophe. Civilizations that had not yet matured enough to defend themselves. To stronger powers, these were opportunities.
And sometimes, invasion had nothing to do with wealth or strategy.
Overpopulation forced expansion.
