Which LEGO theme introduced the transformable robot characters known as Toa?

LEGO Bionicle introduced the transformable heroic characters known as Toa.

Bionicle launched in 2001 as a science-fiction and fantasy theme set in a world populated by biomechanical beings. The Toa were heroic figures associated with elemental powers, while the story also featured Matoran villagers, the villainous Makuta and numerous masks with special abilities.

The products differed from ordinary LEGO System sets because they used specialised parts, ball-and-socket joints, masks and buildable action figures. The theme was supported by comics, books, online material and animated films, making its narrative unusually extensive for a construction-toy line.

Bionicle is sometimes confused with Hero Factory, which replaced it as LEGO’s major buildable-figure theme in 2010. Toa are specifically Bionicle characters; Hero Factory used heroes, villains and robotic machines instead.