Link to video containing location of Mysterious Cards, also a (Spoiler Heavily) text description of roughly where the cards are.
Mysterious Card 1 : 0:00
Mysterious Card 2 : 2:45
Mysterious Card 3 : 3:58
Mysterious Card 4 : 4:53
Mysterious Card 5 : 5:57
Mysterious Card 6 : 6:52
Mysterious Card 7 : 8:40
Mysterious Card 8 : 9:41
Mysterious Card 9 : 11:52
Mysterious Card 10 : 12:22
Voice of Cards The Isle Dragon Roars All Mysterious Cards Locations (text guide)
The first card is obtained as part of the story when you help the old lady to the apothecary in Nexton.
In Thriceton, there is a man on west side of the town. Give him an antidote to obtain the second card.
In Woodland, the third card can be obtained by giving Ridis’s Mother any item for her illness, even if it doesn’t help. Doing this either before or after using the Inn both work.
In Woodland, after putting the fire out in Ridis’s home, the 4th card can be obtained by giving the old man by the well some salve.
In Unionville, the 5th card can be obtained by giving the Goblin on the top right part of the town some salve, as usual.
In Shoreland, after talking to the old lady in the south side of the town about her missing medicine, you can find the thief by the shore. The 6th card is obtained regardless whether you let the thief keep the medicine or take it back.
After obtaining the Magical Key from musty tower 4th floor, return to Advent to open the locked door. The 7th card is in the chest behind the door.
In Village of the Sick, talk to the man in the north side of the town. The card can be obtained regardless if you give him salve or not.
In the Volcano, after reaching the third floor, there’s another set of stairs going down on the left side of the map, take it down to the floor below and the 9th card is in a chest in that area.
In the Mausoleum, on the 3rd level (with the hidden doors), the card is in a chest in the second layer of doors.
Image of location below: