Tarddiad is the original homeworld of the Elves. It was the first world visited by Seren after tending to Mah on Freneskae. She was enamoured by the Elven culture and stayed with them, teaching them the art of crystal singing.
The Cywir Clan, valuing traditional beliefs, stayed behind, but regretted their decision. Without Seren, the majority of the clan died from a feeling of loneliness and despair until they found crystals with Seren's essence. Using a variant of the Song of Creation, they imbued them onto their bodies to repress their feelings and found that they could manipulate their bodies. However, this came at a cost, as it resulted in a need to gain more crystals. This resulted in many aspects of traditional elven civilisation being abandoned, and the few sane members became nomads. Those who were too obsessed with getting crystals turned into Crystal Shapeshifters.
The landscape of Tarddiad is described by the World Gate as "a world covered entirely by trees." The plane also includes various crystalline features. Tarddiad is visited using the World Gate during The Light Within, and remains accessible afterwards. During the quest, the player must collect 50 Tarddian crystals to reconstruct Seren by defeating Crystal Shapeshifters and/or collecting crystal fragments for Angof.
Solid crystals, branching crystals, and floating crystals can be harvested to gain crystal fragments, which can be traded with Angof for Tarddian crystals at a 3:1 ratio. Alternatively, the Crystal Shapeshifters can be fought to obtain Tarddian crystals directly. These can be used to buy both crystal armour and its attuned version. 750 Tarddian crystals are required for the basic version, and 5000 Tarddian crystals are required for the attuned set, which can both be bought from Angof's Crystal Armour Shop.
Hidden around Tarddiad are three journals that fill in some backstory about Seren's arrival on Tarddiad. East of the World Gate, on the west side of the stairs near the southernmost ruins, is a crystal formation that can be searched for the Dusty Tarddian Journal. In the northwest area is a large boulder inside a house that can be searched for the Ancient Tarddian Journal. Northeast of that, next to the river, is a bush that can be searched for the Weathered Tarddian Journal.
^Unknown author, The Book of Seren, RuneScape. "The dawning of our race was a timeless period, so rapt were we with our home and our goddess, but all changed when the green god - Guthix - visited our world. He invited us to join him on a journey to a perfect world."