As an additional cost to play Mask of the Mimic, sacrifice a creature.
Search your library for a card with the same name as target nontoken creature and put that card into play. Then shuffle your library.
All that matters for Mask of the Mimic is that there is another Darksteel Colossus in play when you play the Mask. Mask targets a creature in play and lets you search your library for a card with the same name as that creature and put it into play. You do not have to sacrifice an artifact creature specifically. If you controlled, for instance, Merfolk of the Pearl Trident, you could sacrifice that and search your library for a copy of any other creature that happens to be in play and put that into play. If you or your opponent does not currently control a Darksteel Colossus, you will not be able to put one into play from your library using Mask.
You also cannot do this: if you control the only DSK, you announce Mask of the Mimic targeting DSK and then pay the additional cost for Mask by sacrificing the DSK. It will then be shuffled back into your library but Mask will be countered because all its targets are illegal when it tries to resolve (you might want to pull this trick if you had Pandemonium in play, for instance).
