I was guessing darkness because it would explain two things
-the reason for summoning heartless
-His link with Riku who also controls the same element - maybe during the year between KH COM and KH2, Saix could have tought Riku more about controlling/manipulating darkness
-the reason for summoning heartless
-His link with Riku who also controls the same element - maybe during the year between KH COM and KH2, Saix could have tought Riku more about controlling/manipulating darkness