I even came up with a lore for it, but if you want to write a new one I'm totally ok with that
long before the celestials attacked, when terraria's spirit was at its prime, he created sentinels to assist him in his duties. the first of which being arkadium, the purity elemental. arkadium was a loyal servant, doing the spirits bidding. he created many of the creatures we see in the terrarian lands today. but the spirit had not considered that he had created a minion with its own free will, and soon arkadium became greedy. he wanted to be the spirits equal, as the spirit could create intelligent life. arkadium once tried a peaceful approach, simply confronting the spirit about his duties. when he asked for more power the spirit denied him of it, this sparked anger in arkadium, but he kept to himself. as years went on arkadium became bitter and untrusting. he believed that the spirit was planning to replace him. so he used his almighty powers to completely wipe out a city of them, withering and decaying they're very flesh. this caught the attention of not only the spirit, but many humans as well. they formed a cult, calling themselves 'arkadium's reach'. pleased that these humans worshipped him, he took them and morphed them into something better, stronger, digging tunnels through the earth and laying traps. the spirit of terraria was mightily displeased, but if he were to fight arkadium, such a battle would cause pandemonium. so he created a powerful wraith being specially created to combat arkadium, called the reaver. it defeated arkadium and sealed his very essence in a token, and gifting it to the spirit. the spirit kept it with him, even when he was defeated and manifested himself into the land, and the reaver wandered the dungeon, sealing himself there for when it was needed. some say that arkadium survived the encounter, and his token found by a wanderer. arkadium hides out in the reach... or at least fragments, broken, keeping himself alive only by the thought of vengeance, as he manipulates you so he can free himself