This is not the same bug most folks have experienced with this quest. My problem starts earlier. I'm unable to speak to Arnheir. I am at Level 18 after doing many sidequests and am now getting around to the main questline. I have two dragon souls (only one is required for this part of the quest) but when Arnheir asks me to speak to him,nothing happens. I can't talk to him, I can't equip Unrelenting Force, nothing happens. After searching the fora I haven't seen anybody else with this problem. During Dragon Rising I somehow did not absorb that particular dragon soul; going back to the watchtower scene didn't help. I got my two dragon souls by slaying blood dragons at levels 16 & 17. Any suggestions? I really don't want to go back 15+ levels...

You can try saving, then quiting the game. Reload your last save and see if he talks then. I've had bodies not being lootable and I got around it by saving, quiting, and then reloading the game.

Never mind, I should've RTFM.