After the Emperor's most favored son, Horus, failed in his heresy, the still loyal legions of Space Marines were split into smaller chapters of 1000 marines each.
The still loyal legions of Space Marines were split into smaller chapters of 1000 marines each. This was after the Emperor's most favored son. This Emperor's most favored son was failed in his heresy. The Emperor's most favored son was Horus.