The Knights of Ren

"Even you, master of the Knights of Ren, have never faced such a test." - Snoke | Who are the Knights of Ren?

The knights of Ren are an organization under Snoke, that uses the dark side of the force. They are led by Kylo Ren, and their organization was formed sometime before 34 ABY (After the Battle of Yavin - Star Wars IV A New Hope). The Knights of Ren are not part of the military hierarchy of the First Order and operate independently. The color of Kylo Ren's black and silver battle helmet is according to the color code worn by the Knights of Ren.

In The Force Awakens, Rey saw The Knights of Ren in her vision.

This gives good reason to believe that we may see the Knights of Ren in the upcoming Star Wars movie, The Last Jedi, or at least more about them.

Source: Wookieepedia

Thank you for reading! And as always...

May The Force Be With You!