The Shadowed One could fire beams from his eyes that would disintegrate anything upon contact. He had a staff which he used to create crystalline protodermis around enemies. The staff was also equipped with a Rhotuka Launcher that fired madness Rhotuka, which would cause chaos or insanity in the target's mind.
Umarak the Hunter was basically the ancient creature of shadow and he even had his own Nuva symbol to represent his element. He could use shadows to teleport, create "clones" of himself, behave like a telekinetic force, and to immobilize his enemies. When he became the Destroyer, he was completely under Makuta's control but he also seemed to be impervious to any other elemental except for that of light. He ultimately was absorbed by Makuta so latter could return from the shadow realm.
So which powers do you think we should we use?
Hmm...After he fuses the powers of his Mask of Shadows and Vezon's Mask of Fusion, it could kind of be like how g1 Makuta absorb enemies with their shadow hands.
I don't necessarily think he needs Brutaka's powers but rather some amount of each element to open a "gate", this also gets the Toa more involved like in g2.
Not sure how much you know about g2 so here is a little summery.
g2 Makuta created the Mask of Ultimate Power (MOUP) by combining all the elements into one mask. Ekimu knocked the mask from his brother's face, shattering it. The fragmenting of the mask opened up the Shadow Realm and the whole city was lost to it. In order for anyone to return, all the elements must be used again. Umarak was tasked to acquire the lost pieces of the MOUP and some amount of each element (he used the Toas' weapons).
I was also thinking that maybe Annona should basically be the Nuva of Shadow as she seems to be tier above the Kal. Since shadow is the absence of light, which could have been Mata Nui's element, what better way to imprison such a creature than in a star.