If Narset, Enlightened Master has double strike and trample, then you would gain control of Darksteel Colossus during the first combat damage step (Darksteel Colossus does not have first strike or double strike, so it doesn't deal damage in this step), and Darksteel Colossus would be removed from combat because control of it changes. Then, because no blockers remain for the next combat damage step, you would simply assign all of Narset, Enlightened Master's damage to the defending player.
November 8, 2015 3:38 p.m.
perfect! thank you!