Thresh Tortured Xerath?

From past lore we know that Xerath was trapped with Renekton. To my knowledge we don't know about their release. Could it be that the "warlock who had infused his body with raw sorcery, which gave him the power to regenerate his flesh, no matter how grievous the wound" is Xerath? In his revised lore we are not given a reason for his madness nor additional information about his imprisonment and eventual release (though I believe he was released by Sivir and Cassiopeia). Perhaps it is because of years of endless torture at the hands of the Chain Warden himself which twisted the mind and soul of Xerath.

Xerath was the one soul the lanturn could not contain. Thresh now seeks the cries of pain from the League but also to capture the Magus Ascendant's soul.

If I am missing lore can you forward me links? I Just read Xeraths lore and it didn't make sense to me nor is the lore of Cass, Sivir, or Renekton helpful. We know a little from Azir's but not much about Xerath.