LOL. Yeah, that looks like a boulder spawned on top of the altar. Did you start this world seed before the Hearth & Home update? Apparently some things got shifted on the first H&H patch. Good news is I think you can kill Eikthyr on any world. Start another, kill him there, then relog into your main world. You will still have completed that quest, and best of all, it won't affect the mobs that attack your current base. Each boss killed on a world ups the ante for the attacks you get. One other option is to look around for another altar. For Eikthyr, I've found several possible summoning places.