Putting aside the question of who would play the Executioner, I think that I may have just had the original question answered. Warning, Spoiler Alert!!
Tonight's episode of Marvel's Agents of Shield had a very interesting last 5 minutes. At the return from the commercial break, the final scene showed a brunette walking through Death Valley. She approaches a roadside motel where a newlywed couple is preparing to leave. While the bride is inside, the groom is outside at the car packing up. The brunette walks up to the groom, takes his bottle of water, and downs what remains of his water. He asks her if she is okay, to which she responds (in a British tone) that she is parched, weary, and asks if "all of Midgard is so desolate?" She asks him to take her out of there, but he declines. She asks him about his new bride, and after he states there is no other woman that he prefers, she apparently places a spell on the man before he drives off with her, leaving his new bride back at the motel. As they are driving off, he claims to her that he has vowed to protect her. He asks her name, to which she replies "It's Lorelei."
After doing some homework, I found that Lorelei is one of Enchantress' aliases. However, after doing some more homework, it turns out that Enchantress has a younger sister named Lorelei. And the role was played by actress Elena Satine. The preview for next week's episode showed that Jaimie Alexander will be appearing as Lady Sif. So the cross over episode featuring her that I originally mentioned airs next week. Next week's episode will be one of two parts that will tie into the new Captain America film.