Unlike Gaia, Erebus was very clear on how to target his opponent as much as possible while avoiding destruction to the world.
What use was there in fighting him, when piercing through that layer of 'skin' he was clothed in could truly threaten him?
At that time, one side would be Earth's incarnation, targeted but at least one of their own, and the other side would be the uncontrollable, destructive Outsider. It was glaringly obvious where the present world's instinct would stand.
Thus, Darkness surged from the fissures of the Earth. At first, it was a mere trickle, but in the span of a breath, it became immeasurable. Twisting and flowing, it reached out towards the towering figure standing upon the Earth, as if to envelop the great deity within.
As long as he could make contact, Erebus could use his authority to unveil the 'secrets' hidden beneath and expose them before the present world and the gods.