The Rock
Not to be confused with Dwayne "The Rock" Johnson.
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 under suspicious circumstances towards the end of the Horus Heresy, the fortress-monastery's reinforced foundation meant that its property was the largest remaining piece. 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 in the process of being redeemed (by which we mean they are tortured to death by the Inner Circle of the Chapter), as well as Luther, the original Fallen Angel, 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).