Hordika venom, from the Matoran word meaning "half beast",1 also known as Visorak venom,2 was a substance used by the Visorak to induce dramatic mutations, often resulting in victims resembling half-beast versions of their former selves.
Nature of the Venom
There was speculation that the venom was a dilute form of Energized Protodermis, but no proof was ever found, and the Rahaga voiced doubts that it was true.3
Physical mutations caused by the venom were unpredictable.4
Despite technically being a mutagen,5 the venom differed from other mutagenic substances, namely that it seemed to always cause psychological damage6, 7 while others, like Pit Mutagen, did not.8 This is why Toa infected with the venom lost access to their mask powers9, 10 and the ability to form a Kaita.11 The venom was also different for not being able to affect inorganic objects,12, 13, 14 but Energized Protodermis,15 Pit Mutagen,16, 17, 18, 19 Reconstitute at Random Kanoka,20 and Roodaka's Rhotuka could.21 The only exception was when an object was closely tied to a being, and that being was also mutated by the venom.22, 23, 24
Notes
Krana and Kraata could be infected with the venom because they were organic.25
The effects of the venom could be cured by either removing it or rendering it harmless.26
If a Matoran were exposed to the venom, his or her mask might have fused to his or her face, but it's uncertain given the unpredictable nature of the mutations.27
Keetongu had the only possible cure for Hordika Venom.28, 29
The Ignika could probably cure Hordika Venom.30
A being infected with Horidka Venom that died and was revived on the Red Star would have a new and restored body, but their consciousness would still be affected by the venom.31
Visorak venom would be unlikely to affect Keetongu.32
Sidorak and Roodaka could be mutated by Visorak venom.33