Assassins league Motifs are chapters in the assassins league crafting Style. They drop from various bosses in the Dark Brotherhood DLC. They need Tainted Blood, which is rarely found by killing npc's with the blade of woe, to be crafted.
Bosses that drop the chapters are only the world and delve bosses.
The table for dropped motifs are:
Ironfang: Legs, helmets, shoulders, staves and shields.
Exulus:
boots, bows, Maces, chest and swords
Kvacth arena: Daggers and gloves
Limenauruus: Exes and belts