Seeing that they were brought outside of Mythbreak's interior, the twins looked at Greg with surprise.
They weren't surprised because Greg used his power to bring them here through his shadow, but because of the sight, they saw inside the darkness.
When they were brought inside there, they were able to see hundreds if not thousands of small gates with different scenery moving in them. They instantly realized that each gate represented a shadow from the surroundings, so just the thought that Greg was able to access to all of them, made the twins instantly speechless.
It was a common fact, that even those with the element of darkness could only 'teleport' through the darkness only if they had their destination marked.
Even Kuragari, who was a professional when it came to shadow and darkness control needed to mark the place with his power to be able to use his power on it in the future.