Human: 請將以下內容翻譯成英文，保留原有的HTML標簽，僅翻譯標簽內的文字內容，不新增其他HTML標簽，不新增任何解釋、註釋或其他內容：,Lin Feng went through the other martial arts on the bookshelf, but could not find one that was more suitable for him than the Cross Shadow Killing Technique. Eventually, he gathered all the martial arts on the bookshelf and placed them inside the stone ring.,Lin Feng subtly shook his head. Old Northern didn't mention this huge door to him, and it was clear that Old Northern as well as the predecessors from Cloud Sea Sect never opened this door before.。