Complex sentence: In alchemy, "Caput Mortuum" signified a useless substance left over from a chemical operation such as sublimation; alchemists represented this residue with a stylized human skull, a literal death's head.
Split into simpler sentences: In alchemy, it signified a useless substance left over from a chemical operation such as sublimation. Alchemists represented this residue with a stylized human skull, a literal death's head.