The Illidari-Bane questline is a series of quests that leads to some cool anti-demon weapons. It involves entering many level-70 instances, so you probably want to do the prerequisites before then. David Wayne is also rumored to be Mograine of the Ashbringer.
Quest Line
1 - http://www.wowhead.com/?quest=10621 - Illidari-Bane Shard - Drops from Overseer Ripsaw.
2 - http://www.wowhead.com/?quest=10626 - Capture the Weapons - Soloable as a warlock with banish.
3[A] - http://www.wowhead.com/?quest=10662 - The Hermit Smith
3[H] - http://www.wowhead.com/?quest=10663 - The Hermit Smith
4 - http://www.wowhead.com/?quest=10664 - Additional Materials
After returning Additional Materials, he offers you two quests which can be completed in any order.
5 - http://www.wowhead.com/?quest=10665 - Fresh From the Mechanar - The Mechanar in Tempest Keep
6 - http://www.wowhead.com/?quest=10666 - The Lexicon Demonica - Shadow Labyrinth in Auchindoun
After returning both of these quests, he offers you the second pair which can be completed in any order.
7 - http://www.wowhead.com/?quest=10670 - Tear of the Earthmother - Shattered Halls in Hellfire Citadel
8 - http://www.wowhead.com/?quest=10667 - Underworld Loam - The Steamvault in Coilfang Reservoir
After finishing all these, you can get your new weapon!
9 - http://www.wowhead.com/?quest=10676 - Bane of the Illidari
10 - http://www.wowhead.com/?quest=10679 - Quenching the Blade
The rewards are:
http://www.wowhead.com/?item=30788 - Melee 1H Sword
http://www.wowhead.com/?item=30789 - Melee 2H Sword
http://www.wowhead.com/?item=30787 - Caster Dagger
http://www.wowhead.com/?item=31745 - Melee Dagger
Combined with the http://www.wowhead.com/?item=30756 that started the quest chain, you will have a lot of extra attack power or spell damage versus demons.