RUINS OF CALIBAN
Caliban originally had a very cruel and harsh environment. However civilisation of man flourished and was concentrated within great cities led by a knightly order simply known as the "Order". The Dark Angels primarch, Lion El Jonson's primarch capsule landed heer in the jungles, and he spent his early years within the jungle, doubtless, surviving the vicious predators due to his primarch qualities. After being found and taken to the capital, and being adopted by the co-consul, he pacified the border tribes and brought peace to the world. After the assassination of his adopted father, he took over as the consul and subsequently joined the emperor on his crusade, the planet becoming the home of Dark Angels.
After the death of Horus and the incarceration of the emperor in the golden throne, Jonson had to face the triple betrayal by Luthor, fighting him in a cataclysmic combat, at then end of which caliban was torn asunder by the Dark Gods, the remnant, housing their main fortress monastary, remains the home of the Dark Angels today, known as the Rock.
(Ref: Codex Angels of Death, 2nd ed;)