While it won't affect this review, going to bring it up now, I hate spiders, of any kind. Its actually for that reason that I remember not liking BIONICLE 3 as much as the first two (and is also one of the two reasons why I avoid re watching Harry Potter and the Chamber of Secrets to this day... I see you laughing there, any day you want to stop).
One of the problems with a prequel sequel is that it can take away any sense of shock value. While it works for Legends of Metru Nui as, at the time, you could be forgiven for not making the connection that the Toa Metru are the Turaga from the Mata Nui Saga. However this becomes a problem for Web of Shadows as it serves as the bridge between Legends of Metru Nui, and the start of the Mata Nui Saga, Quest for the Masks. I've probably already spoiled it there but encase you want to know how it does so, time for the review of BIONICLE 3: Web of Shadows.
With the Makuta defeated and right before the last scene of the last movie, the Toa Metru return to Metru Nui to discover the city, in such a small amount of time mind you, is now in ruins, both from what Teridax did and from what has now arrived on the island, the Visorak Horde... I hope you don't have Arachnophobia because... well, lets just say that there's a reason why its called the Visorak Horde. Ruled by Sidorak and Roodaka, the thousands of mostly spider like Rahi (only 4 legs... Its hard to make 8 legged spiders out of Bionicle pieces, trust me, I've tried...
With all the Matoran still in the Colosium, where Sidorak and Roodaka now call their palace, the Toa Metru travel to the center of Metru Nui... Only to be captured by the Visorak and mutated into Toa Hordika, half Toa, half Rahi. With the aid of the Rahagah, half Rahkshi, half Toa Hagah (look up them on your own accord should you wish to), most of the Toa Hordika begin their search for Keetongu, a creature who can cure them of the Hordika venom. Notice how I said most? Well it turns out that Vakarma in his Hordika body can easily be manipulated, and sides with Roodaka. Upon finding Keetongu, the rest of the Toa Hordika attempt to storm the Colosseum, Matau brings Vakarma back to their side, Keetongu kills Sidorak and, in a face palm moment, the Toa kill Roodaka, but free Teridax in the process. I do mean kill for these, as far as I'm aware with the lore of the series, Sidorak and Roodaka are gone now, they ain't coming back. The Toa are returned to Toa Metru, save the rest of the Matoran and we close the Metru Nui Saga.