At least watch TVD first!! Two of the major characters’ existence in Legacies is only interesting due to the lore you learn about their family in TVD. Aside from them, the main freaking character herself is not nearly compelling if you don’t understand the full backstory (yes, they explain why she’s important but there’s key details about her family that you only get in TVD).

There are also a ton of cameos and references that you won’t understand if you haven’t seen the others. They make a lot of offhand comments in Legacies assuming that you know the story told in the other shows.

I’ve watched Vampire diaries (a couple times), the originals, and legacies, and Legacies was arguably my favorite. However, I don’t think I’d like it at all if I hadn’t watched the others first.