Spent half an hour backtracking my way through the Devourer of Souls quest line. If like me, you previously "may" have picked it up but abandoned the quest whilst levelling, then it doesn't reset to the first original quest giver, but from the questgiver where you left off.
If you picked it up from UC, then it would be Carendin Halgar -> Godrick Farsan -> Hearts of the Pure quest. This one is fairly straightforward.
If you picked it up from OG, then it would be Gan'rul Bloodeye -> Cazul -> Zanjaka -> Gazrog -> Ken'zigla -> Grunt Logmar -> Grunt Dogran... and so on... and on... Try going and speaking to each of them in sequence to work out where you left off. Use wowwikia to help you track it. http://wowwiki.wikia.com/wiki/Quest:Devourer_of_Souls_(Orgrimmar)
