Vestige

A vestige is something referenced on Kasdris Plaque. They seem to somehow be tied to Praeteris and the Vel'Karis. According to the plaque, they seem to be extremely dangerous somehow.
As of yet their exact nature is entirely unknown, though Thaddeus Hargrave revealed that they meant to be the strings of a magical harp called The Chains of Phantasia.
Arabella Whitby is actively hunting the Vestiges, and Lucien Novelle believes that he and some of his old friends were doing the same, using a Vestige Compass, which he has since reacquired. 
They posses strange features, such as their ability to take different forms and move on their own. Their level of sentience is unknown, if it exists at all, but regardless, their inability to be tracked reliably has been Seras greatest frustration for well over a century.
The Vestiges have all been purified by the 'Group Alpha'
Red Vestige
Black Vestige
Gold Vestige
Purple Vestige
Green Vestige
Silver Vestige
Yellow Vestige
=== Related ===
The Purifier
The Chains of Phantasia
Nemesis, The God Killer Blade
Kasdris Plaque
Praeteris