Ah yes, the Night Lords - the Legion who, before their degradation picked up speed, was amongst the most harsh to those who didn’t fit in with the growing Imperium of man. Why, their Index Astartes article even mentioned how rebellious systems would surrender the moment they understood that the Eight Legion were coming for them, and put to death all psykers, abhumans, mutants and renegades.
Here’s what FW’s 9th HH book has to say about their conduct in the years and decades leading up to the Heresy:
“Rather than allow the Night Haunter's unpredictable madness to draw the Legion into disaster, the veterans of the old Legion sought to make use of the Midnight Treaties. These hidden texts were bargains struck by Curze during the Great Crusade with certain factions along the fringes of human space, factions that did not fit within the tidy bounds of the Emperor's future, but that could still offer the Night Lords power: abhuman breeds that skirted the boundaries of humanity, reavers and madmen that preyed upon their fellows and Forge Worlds whose doctrines were unacceptable to Mars. All these and more had been concealed by the Night Haunter in return for their loyalty, unnoticed along the dark edges of the maps created by the Great Crusade. Small scout craft, crewed by the most loyal and captained by the most trusted of the Legion, were sent forth to distant and hidden stars. Many of these ships were lost, some crippled by the swelling warp storms that scoured the area, others intercepted and destroyed by the fast cutters of the Dark Angels or Thramassi cruisers, but some few would reach their destinations and issue the Night Lords' summons. “