spoiler
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do you know the dreadful feeling and oppression that ordinary ppl and lower ranks feel when an S-class hunter is around? some people even faint or get sick because their whole being is panicking and freaking out feeling the danger of higher ranks
in this case, the S-class hunters will feel the same the moment MC unlocked his power, even tho it was only barely 10% of his original power. ogKimGiryeo was supposed to have a second awakening, he's supposed to be SS-class. even alienMC feel bad because he really like the owner of the body (he will finally unlock his memory after some event), he describe him as a "good soul". alienMC thought, imagine the glory ogKimGiryeo could have enjoyed if he didn't die.
he died of extreme depression, aside that he already got lung cancer (he didn't know he got it but that was not the main cause).
the depression is also the cause of why MC cannot remember all OgGiryeo's memory, he's suffering some brain damage
he died of starvation because due to extreme depression, it's just one day he stop caring, he can't eat food since his stomach no longer functions properly, if he try to force food, he gets bloated and get stomach ache. days before he died, he's just sleeping most of the day, not eating or drinking anything, when he got the energy he will getup and take a bath
remember why he's wearing a suit the day alienMC got in his body? it was the clothes he plans to wear for interview after passing civil service exam
when alienMC finally unlocked all the memory, he deduced that the death was partly suicide

would it still be long until giryeo obtains his former power?