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The Frostburn rim was obtainable in Season 19: "Freezing Cold" by reaching level 9.
Frostburn is an extremely rim to spot in trading, with it only having a relatively low MU count of ~90 and only 5.8K copies being made (which is similar to that of Wagonwheel, another extremely rare rim). Its also a hoarded item, which means its harder to spot than other extremely rare items.
Top 2 holders control
Largest holder · 116% of circulation