Halo 5: Guardians January Content Update is titled Infinity’s Armory! Scheduled to release at the end of the month.
Back in December a teaser was released that showed off the MARK V armor set, some new weapon skins, and a Covenant Carbine that fired needler rounds. You can check it out here.
343 also revealed this screenshot of a new map in the updated and teased the following concept art for a game type that will be coming in the future.