It's so obvious.
The Primus was renowned for his weapon crafting ability. The Runecarver was used by the Jailer to make (something), presumably weapons.
The Primus has been gone a very long time. The Runecarver has been imprisoned so long that he doesn't even know.
The Runecarver has had his memory wiped so he wouldn't obviously give away the fact that his true identity is the Primus.
They even look similar, and even though they both cover their faces its even more obvious to obscure both bits of information if the big reveal is that the Primus is the Runecrafter.
The Primus must have worked alongside the Jailer, like Denathrius is, until he was betrayed by him (or the Primus betrayed the Jailer) and got jailed up for us to find.