Quote Originally Posted by DaveTycho View Post
No one knows yet why the Night Lords were sent to Isstvan. Maybe ADB will explain it when he does the Night Lords Heresy novel. My opinion though is that while they were considered crazy, possibly criminal too, the Night Lords were never considered traitors, and that's why they were sent. Could also be that when the call came out to prosecute Horus, they were one of the 7 Legions who put their hands up and said "I'll do it!" while other legions were too busy/ far away/ themselves getting massacred
This is most likely the reason. Horus has engineered it so certain legions were well out of the way, whilst others were close to hand, namely the ones supporting him and some loyal ones he wanted to kill. The emperor probably just grabbed those who were closest and this who could most quickly gather their full strength and sent them.