A genius modder has made a festive mod for Mass Effect 3. Not sure if it's only for the Citadel DLC (all the pictures for the mod are shown in it), but either way, it's a magnificent beast.

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For more information and a link for download, head to http://girlplaysgame.tumblr.com/post/68192520606/im-commander-shepard-and-this-is-my-favorite