魔道祖师/The Founder of Diabolism (title of this work) refers to the “demonic/devil path”, which is divided into demons/devils as known in Chinese mythology, path as known as “principle” or “belief”, also meant to denote different paths of cultivation.
前世/previous life – refers to the concept of reincarnation in Taoism, where people live through multiple lives
死无全尸/now in death, his body was broken and lost bears a reference to a taboo surrounding death and funeral rites – in Chinese folk culture the belief is that a loved one’s body must remain intact and that burial brings peace to the deceased, hence the preference for inhumation/interment.
曾掀起腥风血雨/once caused storms of blood and waves of death, literal translation “a rotting fishy stink wind blood rain” emphasizes the danger of a massacre or widespread killing by describing the fishy smell/scent of iron in the air, and the rain being replaced with blood rain. The reference comes from Water Margin (水浒传), a famous novel written by Shi Nai’an.
断袖/cutsleeve is derived from the story of the Emperor Ai of Han who famously cut his sleeve so he would not disturb his sleeping lover Dong Xian. Emperor Ai was well known for being homosexual – traditional historians referred to his relationship with Dong Xian as “the passion of the cut sleeve/断袖之癖“.
修仙/[omitted, translates to cultivation path of immortality] refers to meditation, practice of asceticism or cultivation practice (often encountered in Taoism) according to Wiktionary. Here is the Wikipedia section on physical cultivation in Taoism. In this case, the cultivation path leads to immortality, which is also mentioned in the Wikipedia article on Taoist physical cultivation.
鬼道/Path of ghosts means literally “ghost” (鬼) + “path” (道), refers to what Wei Wuxian uses as opposed to 修仙. 鬼 are the types of monsters he typically controls (which are ghosts), while 道 can also be equivalent to “road” or “way.”
十方恶霸/ruthless beyond measure, literally translated as “ten directions of evil barbarism”.
九州奇侠/have the heroes of the land on your side – literally translated as the heroes of the Nine Provinces. The Nine Provinces is an archaic name for China, hence using the word “land”. The Nine Provinces refers to the region before the unification into China – Baidu link here and Wikipedia link here.
高岭之花/[omitted due to redundancy, translates to “The Flower of Gaoling”] is used to describe things that are untouchable and can only be seen from a distance, or things that can only be longed for but are out of reach for oneself (according to Baidu)