In the lore tab for Nezarec's Sin, he is not only called the "darkest hour" but also the "purest light". Mithrax turned the dark powers of Nezarec into a cup of coffee, but not his light. It's possible that Nezarec will return as a powerful wielder of the light that specializes in void abilities.
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Suspect Nezarec is actually one of those light disrupting wisp creatures the drifter described. However he is able to hold a physical form by possessing other creatures bodies. This would explain why he has multiple tombs and has died multiple times. It's not actually him but his chosen physical vessel who died. Additionally this would explain why mithrax looks like he is struggling when researching the relics and why mithrax decided to distill Nezarecs essence and then give it to Osiris. Also note that in the cut scene the liquid vaporizes and fills Osiris body. Think about it, these 2 could easily bring down the traveler and city in preparation for the Witnesss arrival. Mithrax using his splicer gauntlet could take down all the differences while Osiris could perform a ritual to corrupt the traveler with darkness, which would prevent it from running.