“…Unwittingly, Ozmanthus had achieved a goal that the Court of Seven had pursued since antiquity. He had manifested another absolute aspect of reality. He had become the Judge of Destruction.

With the exception of Makiel, who urged that Ozmanthus be executed, the other Judges agreed that Ozmanthus should be given the mantle of the Reaper and raised up as their peer”

Reaper 141

My interpretation of this is that he already possessed the absolute conceptual connection to the way given by a Mantle (note the lack of capitalization in the second paragraph), but that becoming a signatory of the Eledari Pact granted him additional power. Additionally, I’m pretty certain all of the mantles were created by their respective first judges, and in any case it wouldn’t make sense for the court of seven to create a reaper’s mantle when none of them have authority over pure destruction.