In the movie, Dune, we see a large group of Sardaukar soldiers anticipate a battle ritual, where priests collect blood from crucified dead people and paint blood onto Sardaukar foreheads.
Who are those dead people?
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Those are Sardaukar who didn't succeed, apparently.
the Sardaukar troops are the most formidable professional military in the universe in that time. They are secretly trained on the inhospitable Imperial prison planet Salusa Secundus, and the harsh conditions there ensure that only the strongest and most "ferocious" men survive. Sardaukar training emphasizes ruthlessness, near-suicidal disregard for personal safety, and the use of cruelty as a standard weapon in order to weaken opponents with terror.
See also https://dune.fandom.com/wiki/Sardaukar (beware: spoilers for the rest of Dune's story); https://www.denofgeek.com/movies/dune-the-sardaukar-are-scarier-than-you-realize/ , etc.