Legacy is the glowing cosmetic similar to the blighted cosmetics, but the key difference is they are basically a glowing texture overlayed on the default skins. The 4 killers and 5 survivors that have legacy skins are Trapper, Hillbilly, Wraith, Nurse, Dwight, Jake, Claudette, Meg, and Nea. These were the only killers and survivors in the game at the time.

Legacy was a gift, given to players who had prestiged before the date of November 24th 2016, to reward those who had managed to prestige their characters under the old bloodweb system. Which was an absolute nightmare. If you see a legacy skin, that means one of two things. Either they've been playing the game for at least 3 years, or they hacked the game to attain the skins illegally. As this screenshot shows, my Legacy 3 Trapper is legit.

I took this the day I got my prestige 3 Trapper. This was right before the Legacy deadline, and up until that point I hadn't prestiged at all because it was a MASSIVE grind. But I managed to quickly get Trapper to P3 in time.