Chapter 364 - Sam vs Gustav

If someone focuses clearly then they can see that the nails of Gustav already turn dark. Most likely he is already ready to use the dark spiritual energy. With his spiritual vision Sam can see that Gustav is releasing more and more dark spiritual energy. Not only that you also coated his Sword with that. Most likely he was planning to injure Sam with that. Because the moment dark spiritual energy will get contact with anyone it will try to attack them internally and most likely Gustav was searching for that moment. 

Previously when fighting with Tony, Gustav used this move and because of this, he was able to win against him. Fortunately is unable to absorb that dark spiritual energy from Tony's body.