If I remember right it's something like a human spirit (or ghost), he/she (can't remember if it's male or female) was once a guinea pig in the ancient laboratory I think, and his/her appearance is something like Sadako (I guess?)
ummm maybe dangerous is not the words for it... it's something like scared(?) The ghost/spirit is scary, like when you're watching horror movie and then scared at the presence of the ghost, something like that maybe(?) So probably the priest are just scared I think. Also in this manhwa mage and priest are sensitive to spirit like being I think...if I remember right...but don't worry the MC are too OP and even he then grabbed the ghost hair like it was nothing. You can read chapter 86 and 87 of the novel for more details (https://www.readlightnovel.org/m-e-m-o-r-i-z-e/chapter-86) it's been long time since I read it, probably I missed something while I explained it
@Talh what if her best life is in this parallel world? I mean, her luck maybe said: "well in real world I can't make you win lottery or smth but in this world I can give you OP powers and extreme dungeon finder abilities" so then everything would make sense.
If she has 100 luck how did she get stuck in this crazy world to begin with? It was basically a death game in the beginning and this sword and magic world is full of death too. That's some really bad luck if you ask me.