Baldur's Gate 3 has you traveling around Faerun working with companions to ensure that the Mind Flayers don't take over the Sword Coast. However, if you want to work against the Sword Coast, you can ensure that they are able to take over.
Baldur’s Gate 3: How To Feed The Mind Flayer In Rivington Windmill
Once you reach Rivington in Baldur's Gate 3, you can meet a Mind Flayer in the city's windmill. Here's how you can help him out.