A Court of Thorns and Roses Wiki

The blight is cruel like that.

Tamlin, A Court of Thorns and Roses

The blight is how Tamlin introduces Feyre Archeron to the curse that affected all of Prythian when Amarantha stole a large part of the powers of the High Lords of the Courts.

For a time Feyre believes that the blight is something like a disease that affects Prythian by taking the magic away from the High Lords of the various courts, weakening borders, and causing havoc for the last 49 years. However, it is later revealed from one of Tamlin's servants, Alis, that the source of the blight isn't an illness as such, but is actually Amarantha.