their worlds identify method does not include skill text, just the name, so the full effects of unique skills can only be identified by trial an error.
MC has 2 unique skills that no one knows what they do. 1 is a passive effect that takes damage in place of everyone else he considers an ally (he has been the only one taking potions or getting healed as a result, which they are not realizing the significance of), the other is an active skill an for its effect well wait and see. 😜
The harem dude/Hero is spamming a unique skill of his own not knowing it costs health to use, next time they go into the dungeon it is a near TPK from 1 mob 10 floors before what the party is supposed to be able to handle, because the hero goes down from the pain (he is not use to pain at all, due to never taking damage) and the rest collapses as a result, but no one dies (at least in the WN)