Aela of the Companions gave me the "Striking the Heart" quest. She told me to go to Valthiem Keep. I thought this was odd because I had already cleared out Valtheim Keep for the radiant bandit quest given to me by Whiterun's Jarl. Indeed, my map agreed that Valtheim Keep had been cleared. When I went there for this quest, it was completely empty. There was no NPCs, but the loot had respawned at the level when I first cleared it.

This is a truly crippling bug, because I can not progress in the Companions quest line! Is there any console command I can use to make the right NPC appear in front of me so that I can kill him?

See screenshots which confirm all of this:

https://docs.google.com/open?id=0B-62wZPzEQOlODUzMmI1MDMtMmE5OS00MmE0LThiZjQtYzdkZTg5ZDg1MWYx
https://docs.google.com/open?id=0B-62wZPzEQOlYTk5YTNmNWUtMTBkOS00NmFiLTk4YzgtN2Q5Zjg2ODQ3MTU0
https://docs.google.com/open?id=0B-62wZPzEQOlMWI2NjZkZDMtZWNiMC00ZjBiLTg3MGQtNDhiZjNiZTM5MTZh

I have this exact same bug. Cleared Valtheim Keep for the Bandit quest, and it was empty save for new loot. The quest marker showed, set to the middle of the bridge between the 2 towers, but no NPC was available. Once before I had a similar thing happen with Legate Rikke at an Imperial Camp because she had run off to fight a dragon then took her time getting back. But I've been back multiple times, no evidence of dragon activity or dead NPC. Any help would be appreciated.

(I've tried the resetquest CR09 console command suggested in uespwiki, also setstage CR09 20. These didn't seem to help, Aela still thinks I have a quest outstanding and won't give me another one, neither will Farkas or Vilkas as they think I'm already on a task. After the setstage command, the quest marker vanished, but Aelas still thinks I'm on the quest).

Hi,

According to uesp (http://www.uesp.net/wiki/Skyrim:Valtheim_Keep bug section), it seems there's no way to continue companion questline if you encounter this bug.
As mentioned in another thread with similar problems, if you manage to discover bandit's leader id (for example his body is still on the floor) perhaps you can resurrect him via console and kill him again... doing so could make the quest progress.

Otherwise the only way is to reload a previous save and try getting this quest again until you get a not-cleared location (quest location is randomized).

Ah, thanks. I'll have to get Glory of the Dead achieve on another character then, too many hours added since that quest was received!!