Fiendish Friends Quest
In Sanctuary Dungeon you will find a small elven camp at the southern end. There you will find Elder Brae the Wise; he will give you the quest chain.
The first task of Fiendish Friends is to kill 50 Imps. Many can be found around the Hedge Maze.
Go back to Elder Brae and you will move on to the next stage of the chain, Cracking the Whip I. He will request a Stout Whip. These are sometimes dropped by Orc Lords, it is best to collect one straight after killing the Imps before returning to Sanctuary.
For the final stage, Cracking the Whip II, you must defeat an Arcane Daemon. Many are located at the Exodus Dungeon entrance.
Finally, return to Elder Brae for your reward: The Summon Fiend spell. Note that even if you are able to obtain the scroll elsewhere, you must complete this quest before you may cast it.