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Start with Book 1 — Level Up or Die!
Proving Grounds is the fourth book of Underworld by Apollos Thorne, a progression fantasy series, published in 2020. It follows Sanctuary, carrying the series forward. Elorion's fight continues — deeper into the Underworld, stronger magic, and the uneasy bargain with his master evolving. It's available on Kindle Unlimited and as an audiobook.
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