They are [[Splinter]]s{{17s ref|4254|post|66703|Are Honorspren Splinters, or do they hold Splinters?|date=05 October 2013}}, presumably (at least mostly) of Honor. They are part of the magic of the [[Windrunner]]s, an Order of the [[Knights Radiant]].
[[Syl]] says she is an honorspren.{{qa ref|836|26|Is Syl a bonding spren?|date=Sep 22nd, 2012}}