Summons Unique Dragon's soul. Normally they have a spell tome of own.
A chest is placed at the right side of Riverwood entrance for person that have already defeat Dragons or want to get easily.
You can summon Mirmulnir, Alduin, Paathurnax, Nahagliiv, Sahlokni, Viinturuth, Vuliotnaak and Vulthuryol.