Rowan believed that Thenos would have been able to easily steal his powers if he was still at the fourth dimensional level, but when Rowan became a fifth dimensional immortal and carve his own reality that was separated from the reality built by the Primordials, Thenos was no longer able to access his powers.
However, this event could have still gone sideways if Thenos had been able to completely devour the talents of Berrion and not simply swallowed his energy and some of his attributes, because if he had devoured Rowan's Reflection in its entirety, then it would be a backdoor that he would have used to access Rowan's dimension.
It was during moments of examinations like these that Rowan observed the many ways that he could have been defeated, a little slip-up here, a delay there, and the ending of his story would be radically different, and the same thing was happening now, even though everything seemed to be calm and orderly, a single spark could set everything alit.
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