Sith Transformers are evil Robots known to use the dark side of the force to conquer planets and galaxies. They were banned and outlawed on Cybertron and expected to never return again thus making Sith transformers want their evil Revenge against the Autobots. All Sith Transformers joined Evil Darth Grievous's Sith Legion to gang up against the Autobots to fight for control of their homeworld.
In Darth Grievous's evil vision of the Future the Sith transformers were victorious against the Autobots but later sealed in cocoons along with Darth Grievous for later transformation of the Galaxy. After 99 years the cocoons hatched and the Sith Transformers became cape wearing villains and gain giant sombreros on their heads which stayed permanently attached to their heads and turning the sith transformers evil.