Skyrim Darkness has Fallen Episode 18 - Dealing with Daedra Mod - Dealing with Daedra Quest.

Dealing with Daedra
Make a Daedric deal for the secret of ritual magic, discover the true cost. Mutually exclusive with Apex Predator.
Started from a note (on the on the Sleeping Giant Inn door will start Ritual Magic but it will get you taken Hostage ad forced into the Syndicate)
Start at the Frozen Hearth Inn a conversation with Nelacar. Designed to be started at level 1 and played in parallel with a normal playthrough until about level 12 or 25 depending on choices.
Ritual Magic
Individuals with talent but limited natural magicka reserves have long used soul gems, enchanted items, potions, scrolls and staves to supplement their powers. The ways of the
secretive witches are less well understood, but those ritual blood sacrifices and circles of candles everywhere are speculated to fulfil a similar purpose by the members of the Synod. If only someone could gain access to their secrets.