Although Leon only had a single slime's core, he had several golems' cores, and those belonged to bosses. However, he hid them outside New York. They could be sold for a high price, but Leon would use them without any hesitation to strengthen his new card, but since he didn't have them, he couldn't use it.
"Maybe, I can use the skeletons' dust..."
Leon tried to absorb that, but he ended up absorbing only the particles he touched. The amount of mana recovered was low, but he was doing that to train his skill anyway. However, he decided to stop when the particles of bones became lifeless like sand.
"... What did you do, master?" Gisela asked, showing a surprised expression.
"Just a little experiment... don't worry about it; it is not something the current you can replicate," Leon answered.