Bell of Dethmoora is part of the warlock epic mount questline, in which you acquire a Dreadsteed. It was previously only available to a Warlock who completed a long quest chain, but was changed in Patch 3.0.3 to be available to all warlocks at level 61 from the mount trainer.
A Bell of Dethmoora, named after the late warlock Dethmoora Darkeyes, is one of the three tools needed to create a Circle of Greater Summoning. When properly atuned with the Wheel and the Candle, these items conduct vast stores of arcane power.
My servant Gorzeeki can construct such a bell, but to do so he will need a large supply of elixirs of shadow power. Procure the elixirs from an alchemist and bring them to Gorzeeki.
Progress
You have those elixirs, <name>? Mor'zul told me you need a Bell of Dethmoora, and making one of those takes a lot of shadow power!
Completion
Very good! I hope these elixirs weren't too hard to get. Sometimes alchemists can be so miserly with their creations...
I'll make the bell and keep it until you're ready for your ritual.
Rewards
You will be rewarded with the Bell of Dethmoora. The bell will not show up in your inventory, but instead will be available when attempting to complete the quest Dreadsteed of Xoroth.
In addition, once both of the introductory questlines are done, the following will be available (Note: The materials quests are not required for this):