From the Chronicles of Takua…

The masks of Mata Nui can be a source of great power… and of great danger. For the darkness of an infected mask can have grave effects on the wearer, turning him into a servant of Makuta. You can recognize these dangerous infected masks by their blackened and pitted condition – if you value your freedom, you will stay away.

The first infected masks appeared on Mata Nui shortly after the Matoran arrived on the island. Rahi, who had previously been no threat to the koro, suddenly became a danger to the villages and people. It was Turaga Vakama who discovered that the Rahi had been fitted with infected Kanohi that made them obey Makuta’s will.

But where was the infection coming from? It was only after a great, secret hunt all over the island that the Turaga discovered kraata were spreading this plague of shadows. It is only in recent days that we Matoran learned this, or that the Turaga were capturing the kraata and keeping them imprisoned in a cave in Po-Wahi.

A kraata carries the will and darkness of Makuta inside it. One touch of a kraata can turn any Kanohi mask into a tool of the Master of Shadows. Even a Toa cannot resist the power of an infected mask! The only way to help someone who wears such a mask is to remove it as quickly as possible and replace it with a pure mask.

At first, it seemed to be impossible for an infected Kanohi mask to be restored to its original state. The Turaga tried many different ways to do this, but all failed. Later, the Toa discovered that they could use their elemental energies to cleanse a mask, but it requires a huge amount of effort due to the sheer power of Makuta. For this reason, the Toa cleanse masks only when it is vital that they do so.

The Toa removed dozens of infected masks from the Rahi of Mata Nui. Since there were so many and they would be a danger to anyone who wore them, the Turaga ordered that they be buried deep beneath the surface until such time as they could be cleansed. A small team of Onu-Matoran did the work. Only they and the Turaga know where the masks are hidden, and Onu-Matoran are very good at keeping secrets…