First : Amon could strike his opponent from any direction within the domain.
That was how he had fought Gehrman and Abraham earlier… and how he was fighting Frey now.
Second : inside the Transparent World, Amon was granted limitless aura.
In other words, he could fight without exhaustion, as long as he remained within the barrier.
And that wasn't all.
"Since this is a spatial ability, your teleportation cannot allow you to escape it."
The Transparent World restricted Frey's teleportation to the terrain inside the barrier itself.
He could not teleport beyond it.
Which meant that if Frey wished to escape, he would have to run beyond the domain with his own body.
But the Transparent World moved in accordance with Amon's position.
In other words ..
As long as Amon pursued him at close range, Frey would never be able to escape.
To make matters worse ..
Amon was already faster than Frey.
Escape was impossible.
"You are truly powerful, Nameless' vessel…"
