The Tain was one of the many artifacts of power created in the world of Myth. It was created by the Dwarven smiths of Muirthemne, ordered by Connacht the Wolf. During construction, the Dwarves accidentally unleashed an entity known as Syarkosh the Spider-God to their world.
After the Tain's competition, Connacht led a regiment of the Cath Bruig Empire into the Dire Marsh, to rid the world of the terrible race of the Myrkridia, brought to the world by Moagim. After that, Traval and his fellow Dwarves used it to seal the Spider-God, sacrificing themselves to do so. Some users after becoming emperor, Connacht pledged to his friend Damas to burn the items of power, and hide those which can't be destroyed, including the Tain, to stop him from using them once he returns as the new Leveler.
The Tain reemerged during the last year of the War, as Soulblighter used it to trap a regiment of the Legion, led by Cu Roi and Murgen. After two days inside the Tain, the Legion was able to escape with the sacrifice of the two Avatara, and the Tain shattered as a result.
Sixty years later, Soulblighter, using a shard of it, led The Summoner inti it to resurrect the Myrkridia. Some time later, The Deceiver, now an ally of the Light, used another fragment of it to lead a retinue of the Legion to the Summoner. It is unknown what became of the Tain's shards after that.