This is two sentences spliced together with a comma. It needs end punctuation (a period) after newest victim and the next word capitalized as the start of a new sentence.
Also, however is another of those words that change meaning somewhat depending on punctuation or lack thereof: However you chose to get your novel written, just make sure that it does get written. However, there’s a difference between researching a scene and just wasting time on Pinterest.
In the “writing glitch” example, however needs a comma to separate it from the rest of the sentence. (If you can replace however with on the other hand, it needs to be separated; if you can replace it with something like in whatever way, don’t separate it.)
A demon is trying to haunt his newest victim. However, his victim is oblivious to all his attempts.
Haunt doesn’t seem like quite the right word, but I cannot think of a better one right now.