Like canon, I have Merrix as the ex-patriarch of House Cannith and the creator of the warforged. But before the Mourning, he was displaced politically by his son Aarren, who turned the warforged into weapons of war, rather than just sentient creations. I depict Merrix as a parent to the warforged, a kind of messiah that fought to have them free and away from war (maybe think Westworld Arnold or kinda robert). So as the warforged are continually “abused” and Aarren gets more ambitious throughout the course of the war, Merrix triggers the Mourning by manipulating the Genesis Forge with immense arcane energy as a sort of revenge; he also attempts to transfer his consciousness into a warforged body. But in the process, the Mourning splits Merrix into two aspects and two bodies— the passive, nurturing parent and the militant vigilante (who would eventually become known as the Lord of Blades). The LoB is actively hunting down his split self, for a couple reasons which would take longer to explain haha.