Third Possibility

After a moment of shocked silence, he seemed to realize something. 

He said in a calming tone, "Calm down; I'll die anyway if you don't do it."

Now I was seriously worried. Before I could voice my concerns, Lord Nightblade stepped in to explain, shooting what seemed to be a disdaining glance at her partner.

"Mortal, Nightshade was not exactly honest when he told his soldiers that he had not been permanently injured and his mana core had not been affected. As you may know by now, a mana core is a very sensitive thing, and even though the brutal attack from the two Light Lords had not struck it directly, even the influence of the searing light was enough to nearly cripple him,"

"When they attacked him, he had managed to shift its location in time to avoid being even scratched by that accursed blade of light, but the radiance it emitted nevertheless affected it, such that he would have been in grave danger even if he didn't do the reckless action of grabbing and forcibly keeping the sword inside of him, which gave it even more time to damage his core,"

"In all honesty, even if he had not done so, I believe that the effects would have been nearly the same anyway, as the moment his mana core was affected by light it immediately started to collapse on itself, which led to his state. However, with the aid of the other two Dark Elementals, I managed to stabilize and and trap his core in a fixed state - which also meant that the light was trapped in his core and is currently wreaking havoc on him,"

"I managed to dissolve most of the destructive light in his body, but a trace of it remains in his mana core - which despite being the most potent source of darkness in his body, he is unable to actually control - so he cannot destroy that bit of light in it, which will destabilize his core and eventually kill him,"

After hearing her lengthy explanation, I was perfectly ready to act. I didn't fully understand how there could be light in the most concentrated and powerful part of a being of sheer darkness, but based on her words, it sounded like he would die if I did not do something… whatever it was I could do to affect this situation.

My readiness must have shown in my expression, because Lord Nightblade hastened to assure me that acting immediately was not a good idea.

"Mortal, the severity of this problem cannot be understated; please do not do something rash. Anything related to the mana core is of utmost importance for us Elementals. Although it is a little hard to draw a parallel for you humans, seeing as you lack a similar organ, perhaps it could be likened to a heart, except twice as sensitive,"

"Normally, to fix this problem there would only be two possibilities. One of them would be if he were to successfully advance to a Night Reaper right now, since the process involves condensation of the mana core, which would absolve his problem. However, this would be risky, as he is not quite ready to become a Night Reaper; perhaps given another decade or two he would be nearly assured of success. Moreover, even if he were to succeed in the endeavor, his strength would be permanently affected due to the impurity of his resulting core, something he is unwilling to accept,"

"The other possibility would be cutting open part of his mana core, which might be able to remove the light. However, this is a dangerous procedure requiring at least three Night Reapers for even a chance at success, while failure would irrevocably damage his mana core due to the invasive nature of the technique. The only way to have a guarantee for success would be if a being above a Night Reaper operated on him; otherwise, even if the light were removed there is a high chance his core would still be damaged. Needless to say, there is little chance that such a being would be free to aid him and we are too far from a major city to be able to gather even five Night Reapers before his core falls apart,"

"Fortunately, your existence opens up a third possibility for him to survive and perhaps even benefit from this trial… if you are up to the challenge, of course,"