How do you get the radagon icon?

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Elden Ring Radagon Icon Legendary Talisman Location (Shorter Casting Time)

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How To Get The Radagon Icon In Elden Ring

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Radagon's icon talisman states it shortens casting time. By adding 30 virtual dexterity it supposedly does what it states, but how much quicker are we talking? 2%? 4%? it seems like a waste of a valuable talisman slot in my current setup. Is it even worth it? : r