Seth's darkness specialty had gone towards the spatial area. He loved to use darkness to get spatial advantages and create sub-spaces to transport things or people from a place to the other. There was even those Specters of Darkness, a technique he had developed alone, on which he was even able to change position with a specter.
However, First Shadow specialty laid in another area. She was like a darkness mage and her specialty was cutely named by herself, as Nightmare. She loved to make her darkness element swallow her enemies hopes and crush them with it, throwing them in a deep abyss, where there was no return from.