The main trait of the Demon Lords was that they had many Soul Seed Rings. Soul Seed Rings structured this way, known as Soul Rings for short, could allow Demon Lords to reach their strongest state quickly and inconspicuously within a short period of time whenever they invaded a world.
But now Garen only had one Soul Ring, and according to his estimates, it was highly likely that powerful Demon Lords would have multiple Soul Rings. That way, once they entered a new universe with new rules, they would be able to integrate into the new power system there quickly, using the power of the Soul Rings to level up rapidly and eventually controlling the strongest power there.
That was the true terror of the Demon Lords. They were the real pillar in the fight against the Void Creatures.