Filth

"..Leonis, the necromancer who picked up a slime and became friends with it." Rallaka continued her question, gladly providing more detail.

"And somehow ended up influencing said monster and turning it into a half undead, which would render this situation possible if it was the same creature?" Aria began to piece out the story.

That could be it. Maybe it wasn't just their intuition.

Wasn't the unknown creature just a mutated slime variant?

The reason they hesitated to call it a slime was because of the abnormal characteristic and the fact that the slime species can not become an undead.

They had these things called cores inside their bodies, and that was what acted as the engine to their movements and actions.

Slimes were basically unanimated jellies if not for these cores, and that was the reason why when the slimes die, their cores remain.