They trick the Epic Games Store (EGS) into believing the user's account has legal ownership of specific add-ons.
Epic Games actively monitors for entitlement fraud. In 2022 and 2023, multiple Reddit threads reported permanent account bans for using DLC unlockers on free Epic games. Unlike a VAC ban (which might be game-specific), an Epic ban locks you out of your entire library—including any paid games you own.
acidicoala/ScreamAPI: Legit DLC Unlocker for Epic Online Services