From Lexicanum

The Hearing of the Thousands is a title given to an event in the history of the Imperium that came to head on the planet Nikaea where a trial was held over the governance of psychic powers that was being explored by the Primarch of the Thousand Sons, Magnus the Red.

It became percieved as a legal as well as moral trial over the matter of psychic abilities and sorcery. One side argued that there was nothing inherently evil over the knowledge and the problem lay with its uses. Thus, this became the defence over its use and many saw that through the use of such powers, the Space Marines could use it for the greater good of the Imperium. In contrast, the critics noted that psychic powers were inherently unstable and that many were incapable of the strength of will that was required in controlling them.

According to the Grimoire Hereticus, a number of Librarians attempted to storm out of the hall after they were horrified of the reactionary as well as the misguided direction the debate was taking place. Within the archives of the Blood Ravens, the title to this event is known as the Heart of the Thousands. Furthermore, the Apocrypha of Haidyes was used during the Hearing which later became the maxim of the Blood Ravens Chapter. ¹

Source

• ¹:Dawn of War Omnibus The Trials of Isador (pg 270, 271, 272)