The Shadow Toa were the dark essences of the Toa Mata given independent life by Makuta.1
Notes
Tahu's opponent wore a blackened, pitted mask and wielded a sword which billowed black smoke. It spoke in a sizzling, crackling voice.2
Gali's opponent mirrored her own form, but its body was the muted, sickly brownish-black of an oil slick rather than blue.2
Pohatu's opponent spoke in a gravelly voice.2
Lewa's opponent looked like him, but its mask was pitted and blackened as if by a creeping forest mold. Its skin beneath was the washed-out green of a diseased leaf.2
The strength of Onua's opponent matched his own.2
The shadow Toa were basically black-colored versions of the Toa Mata. They had the same Kanohi and powers as their counterparts.3, 4 They were the Toa's dark, evil parts of themselves given substance by Makuta.5
Shadow Toa were an offshoot of Teridax's mental abilities, they were not an experiment because he didn't experiment on the Toa Mata.6