Good luck with that. I even recall a scan I saw here during Blackest Night that said Nekron isn't evil or good, he just is. Likewise, ALL souls pass through his realm aka Purgatory aka Limbo on their way to their final destination : Heaven or Hell. So "absorbing evil" is going to get Nebulos nowhere.
Nekron kills him.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
Nekron was able to sear the hands of LT with his staff though, and with a good number of the souls that pass through purgatory being evil Nebulos could obtain enough power in the staff to overcome Nekrom. I don't think the battle would be all that easy for either one.
Depending on how much evil energy Nebulos's staff has absorbed prior, I think he could take it pretty easily. Now if he's just arriving in Nekron's domain without having absorbed much, then he's a goner. Remember, Nebulos has awful durability and died when LT crushed a mountain on him after Strange took his staff away.
Are we even sure that Nebulos died though? I think he was just incapacitated. An extradimensional being shouldn't be killed by a falling mountain of rock, especially since he was able to get in a tug of war fight with the Living Tribunal.