A Wraith is similar to the grim reaper but they have a different connotation.

The simple explanation of a wraith is this: a ghost or ghostlike image of someone. (one seen shortly before or after their death.)

Originally, the word "Wraith" is a Scottish word for "ghost", "apparition", or spirit". It usually is a used to refer to a rather unpleasant and/or grotesque version of a human ghost.
The Wraith is often associated with an undead sorcerer or sorceress that failed to become healthy at their deathbed through certain unnatural methods (i.e: Methods that do not follow the established laws of nature, most often trying to break and evade many of these laws through darker and more sinister ways). After the sorcerer or sorceress epically failed to become healthy and alive again with their spells,potions, and/or magics, they die and come back to "life" but as a wraith. Perhaps it would be better to say that they don't "come back" to life, rather, their bodies die and an impression of the soul is left behind, a sort of ghost if you will.
They are seen in dark purple robes that are kind of ripped and carry large scythes. Wraiths can be very dangerous even if they were good sorcerers and sorceresses. Once they are brought back to life by their messed up potions and magics, they become corrupted and evil. If a Wraith is powerful enough, they can possess humans. Wraiths will make their home anywhere that's dark and sinister. Rarely a wraith will make their home in a dark basement but if one found its way into your basement, call a priest, bishop, or saint or go down their with a Bible with a lot friends. Once you're down there, gather in a circle and shout, "Dark interloper, leave this holy house! God has found you! Leave or feel the wrath of Jesus Christ and the Holy Ghost! Their light will repel you, acolyte of evil!" Then cross yourself and you and your friends will be protected and the Wraith will be defeated. You should also take a wooden cross down there too.