The Rock
Not to be confused with Dwayne "The Rock" Johnson or the gay bar that used to be near GW's headquarters (no, seriously).
The Rock, also known as the Tower of Angels, is the mobile headquarters of the Dark Angels Chapter of Space Marines. It used to be a very static headquarters called Aldurukh (Ancient Calibanite for "Rock of Eternity"), back when it was the fortress-monastery on Caliban, but when Caliban was destroyed after the Dark Angels mercilessly bombarded their home planet after Luther roused the entire planet into rebellion against the Lion, taking the Dark Angels garrison force on Caliban with him, when the Lion returned home from the Horus Heresy, the fortress-monastery's reinforced foundation and the fact that its void shields were online meant that it was the largest remaining piece of the planet. The Dark Angels put engines on it and now they fly around the galaxy, recruiting from tribal populations on the worlds they visit.
It also seems to have a ton of dark tech factories, which would explain the ton of plasma weapons and terminator armor they seem to have.
The Rock is home to several Fallen Angels FOUL BETRAYERS FROM THE NINE TRAITOR LEGIONS WHO ARE NOT AFFILIATED WITH THE DARK ANGELS IN ANY WAY in the process of being redeemed (by which we mean they are tortured and made to repent for their treason before being finished off.), as well as Luther, and Lion El'Jonson himself, cared for by the Watchers in the Dark (although the Dark Angels are unaware of their Primarch's continued existence except for Luther, who has long since gone insane, although he constantly rambles about the Lion's impending return.).