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From MC 1.21 onwards, you no longer receive the bad omen effect instantly after killing an illager captain. Instead, an Ominous Potion is dropped, which must be drunk to get the effect. While this adds quality to gameplay, it is illogical from a lore perspective, as it implies that, illagers need your consent to raid a village. Using this datapack, it becomes possible to recapture the thrill of facing the consequences of your actions, and hence gives the player an instant Bad Omen effect upon killing the Illager Captain.
C:\Users\(your name)\AppData\Roaming\.minecraft\saves
Put the datapack file (.zip) in the datapacks folder, and while in game, run /reload. Manually reloading the world also works.
/function toggles:classicomen/function uninstall:classic_bad_omen
Reverts the changes done to bad omen in 1.21.