Never Again! (Horde)

Diathorus the Seeker is at 78.47, 83.88. Inside den at top of Demon Fall Ridge, across first bridge. (He is level 22)
Gorgannon is at 89.74, 76.70. At the end of Demon Fall Canyon (He is also level 22) Melee swings + Knockdown effect, and a Shadowbolt that can be interrupted.

Additional detail on finding Diathorus: turn right as you enter Demonfall Canyon and continue to the western end, where Grom Hellscream's grave is. The path up to the den starts just a few paces north of Grom's grave, and it's marked by glowing red stones. Go all the way up the path, cross the flat open area to the den, enter the den and go down the tunnel; you'll find Diathorus standing behind an infernal.

First kill [npc=17300] for [item=45477] then find the Barrow Den with [npc=6072] for [item=45476].

Locations aren't immediately obvious, especially for the Barrow Den, so coordinates formatted for TomTom / Paste users are below:

/way Ashenvale 84.17 71.78 #1 Start of track up Felfire Hill to Gorgannon
/way Ashenvale 89.66 76.74 #2 Kill Gorgannon for his Flaming Blade
/way Ashenvale 83.04 78.06 #3 Start of path to Barrow Den follow these shiny posts up
/way Ashenvale 78.38 81.84 #4 Entrance to Barrow Den
/way Ashenvale 78.46 83.89 #5 Diathorus the Seeker over bridge in Barrow Den