@strayCat0 has a good straight answer to this question and takes just about all of the sources I'm aware of. So, instead, I'll just do my usual thing and nit-pick on definitions here a little bit. In this case, the definition of the word "ritual."
A ritual actually refers to any form of repeated regular action in association with something. For instance, changing your clothing before going to bed (into pajamas, your underwear, or nude, whatever your preference) instead of just sleeping in whatever you wore throughout the day could be considered a "ritual." So, the definition is actually rather wide in scope.
In a more proper sense, any and all forms of religious services count as a ritual.
Therefore, the "sinister" rituals that you are asking for would be absolutely anything that involves a very bad outcome that was carried out in a ritualistic manner. I would say the the "People's Temple Movement" drinking poisoned kool-aid back in 1978 certainly counts. But, we might be looking into something more repeated and regular than that if you're looking research for your writing. In this case, as strayCat0 said before, the Mayans are definitely going to be your best bet for the human sacrifice type department.