Canonically, Takua never discovered his Av-Matoran power and never used it, so we’ll never know what is was. But… that’s not much fun, and surprisingly, there’s actually an instance in canon of Takua using a power in conjunction with a Toa, that really can’t be explained by the normal power-sets of Toa and Matoran.
In MNOG, Gali creates a mental link with Takua that allows him to see the battle underground. The ability to create a “mental link” never comes up again except in Takanuva’s Blog, where it is again used between Takanuva and Gali. The idea that Toa can create such links is otherwise never addressed, even when it might be helpful.
My just-for-fun headcanon: Takua’s power is the ability to share visual experience with the Toa who activates the power. The Karda Nui Av-Matoran were able to share their memories with their Toa partners, so it’s not so much of a stretch. Granted, the normal rule with Av-Matoran powers is that they require continuous contact with a Toa, but the Av-Matoran powers we saw were pretty quirky. Tanma’s seems to only be useful when partnered with someone who just happens to have a light-based weapon, and Photok’s lightspeed power isn’t really a matter of elementally manipulating light. The ability to share a Toa’s experiences, especially visual ones (because vision comes from light) over a distance doesn’t seem totally farfetched.
What do you think?
