Warframe: The Seven Crimes of Kullervo

Return to Duviri and delve into a sprawling new island dungeon where the tragic Warframe, Kullervo, awaits your redemption. This strange, floating prison obfuscates both Kullervo’s crimes as well as the mysterious (yet familiar) Warden who binds him under lock and key. Explore the prison’s cells to uncover the story of Kullervo’s misdeeds and wield the power of treachery through his trademark daggers and thirst for vengeance.

As a mainline update It contains an exhaustive list of quality of live changes to the game. The main new content is the 53rd Warframe, Kullervo, and his signature weapons along with the chance to encounter his story missions on a new island in Duviri.
Other Duviri changes include Captain Vor's appearance in Undercroft challenge missions for a chance at new arcanes, Kaithe customization with armor and ephemera, 2 new side missions with Paragrimm's Tomes and Lost Belongings objectives, the return of Kaithe races and 15 new decrees.