This is your first quest that is class exclusive. Simply take the Tainted Item to the person noted in the quest text. They are not far away.
Summoning Quests
Summoning of the Imp
There are no longer any quests to summon the imp pets. It is now trainable from the starting zone warlock trainer at level 1. Previously, the person that you delivered the Tainted Item to had a quest for you, to get:
6 Vile Familiar heads by killing and looting Vile Familiars by the cave to the north.
3 Rattlecage Skulls by killing and looting Rattlecage Skeletons on the north side of Deathknell.
4 Wraith Essence and 1 Tainted Wraith Essence by killing and looting Arcane Wraiths and Tainted Arcane Wraith in Falthrien Academy to the west.
The book Powers of the Void, near a Defias tent in the Vinyards across the river.
3 Feather Charms by killing and looting Frostmane Novices in a cave to the southwest.
The reward for this quest was the spell Summon Imp.
This questline ends up teaching you the Ritual of Doom to summon a Doomguard. This is the same Doomguard as is summoned by the Curse of Doom which has a 10% chance to summon the Doomguard, but it always summons a Doomguard in a ritual that need 5 players and randomly sacrifices one of them.
The Summon Felsteed spell no longer requires a quest, and can be learned from a Warlock Trainer at level 20. (Used to be level 40 until patch 2.4.3)
Summoning of the Dreadsteed
This quest is no longer required, as Dreadsteed is a trainable ability available at level 40.
The Summon Dreadsteed spell can be learned from a Warlock Trainer at level 40 and requires Journeyman Riding skill.
The quest allows you to obtain this spell (and the associated Feat of Strength) at level 60. This quest line consists of two independent "lines" (One gathering/crafting line and one adventuring line) that can be done simultaneously.
250 (Optional items: either bought from the Goblin: J'eevee's Jar, Black Lodestone, Xorothian Glyphs, or you can ask another lock who already have these items to run you through DM West for the Dreadsteed of Xoroth quest)
Then do one of the following quests. They are not technically prerequisites, but provide the required quest item for Fragments of the Orb of Orahil. Accepting either quest will make the other unavailable. The final chain rewards have been made identical, so it makes no difference which you accept. NOTE: Neither of these quests have anything to do with your ability to summon either a Felhunter or an Infernal; those are completely separate quest chains(see above).
Impsy, Niby the Almighty's Imp minion wants to create a minion for himself - funny, but you will gain some nice items too. The Questline seems to have three alternative starts in Felwood, Stormwind and Orgrimmar.