Add Terror Boss Monsters for each Terror Zone. These don’t need to be new monsters or new models. Just use the Super Unique Monsters already in existence in the game. Give these “Terror” versions +1 bonus attribute and +2 Minions (if they have minions). Current non-Terror versions can have up to 3 special fixed attributes, and +0 bonus attributes in Normal, +1 in Nightmare, and +2 in Hell, this means the maximum special attributes for a Terror Super Unique in Hell difficulty will be 6 special attributes, in Nightmare 5, and in Normal 4. Number of minions will also still be scaled between difficulty levels, just with +2 for the Terror versions.
More details here.