Pokemon are such innocent and intellient creatures. The very idea of Shadow Pokemon is just a crime against nature; Thomas
Shadow pokemon are heartless pokemon, created by artificial means. Their attacks change to shadow moves like shadow break and shadow rush. However, unlike most heartless, they can slowly regain their hearts if they are with a caring trainer, or if placed in a purification chamber on Pokemon Castle or Greenia. Also, they lack the ability to consume hearts and warp, however they are more portable and are very agressive.
Sidiuos created shadow legendary pokemon and used them against Thomas in their battle, their attacks weakening Thomas enough for Sidious to defeat him, after Thomas captured them. They now are being rid of their darkness. Later Zeus began to turn Pokemon into Shadow Pokemon, including Dawn's Piplup