A "grimly ritualistic event" can be described in many different ways, depending on the specific tone or feeling one wishes to evoke. For example, it could be called a "somber ritual," a "morbid ceremony," a "solemn observance," or a "darkly traditional gathering." Other potential synonyms might include " macabre ceremony," "funereal practice," "morose ritual," or "solemn rite." The key is to choose a term that effectively conveys the sense of seriousness, gravity, and formality inherent in such events, while also respecting the cultural or religious significance they hold for those who participate in them.