Shadow is the type of magic used by Magus. Some of Robo's attacks also have Shadow elements. When two or more elemental attacks are combined, they cause Shadow damage.
When meeting Spekkio for the first time, he will explain that there are four types of magic: Fire, Water, Light, and Shadow. Crono's party has magic in them, which allows Spekkio to grant them magic. Crono (Light), Marle (Ice), Lucca (Fire), Frog (Water), and Magus (Shadow) can all learn magic, while Robo and Ayla cannot, due to Robo not being a life form and Ayla existing before magic was created.
There are three Techs that Magus can learn that are Shadow-based.