I don't recall reading that Dessimbelackis was first sword at all. I thought he was the Emperor/self-proclaimed god/First Nutcase of the first human empire...
...I just check the EM and there's only one quoted paragraph there that has him and the words 'sword' and 'First Empire' in it, and it's a
huge stretch to imagine the speaker means that
he is the actual First Sword of the First Empire;
Quote
Before the appearance of Our Great Emperor, Dessimbelackis, our legions were thrown back on the field of battle, again and again. Our sons and daughters wept blood on the green ground, and the wagon-drums of the enemy came forth in thunder. But no stains could hold upon our faith, and it shone ever fierce, ever defiant. We drew our ranks tall, overlapped shields polished and bright as the red sun, and the one among us who was needed, who was destined to grasp the splashed grip of the First Empire's truthful sword, gave his voice and his strength to lead us in answer to the well-throated rumble of the Sar Trell warcries, the stone-tremble of their wagon-drums.
It's entirely possible that they had no First Sword at that time for whatever reason - no role/no candidate/dead/couldn't be arsed fighting numpty Trell/holidaying in Jacaruku, whatever, and that Dessim rallied them.
It's also entirely possible that the whole account was historical fiction..... wait..... >.>
Yeah, so I don't think the 'Dassem is Dessim reborn' theory holds up tbh.