Radiance is a Runeword "NefSolIth" in Diablo 2 Resurrected (D2R). It requires these 3 runes to be inserted into a socketed item in the exact order: Nef + Sol + Ith . The eligible bases for Radiance are the following: Helms, Barbarian Helms, Circlets, Druid Pelts .
RadianceRadianceGrand Crown
Nef · Sol · Ith
Defense: 199
Durability: 50 of 50
Required Strength: 103
Required Level: 27
+75% Enhanced Defense
+30 Defense Vs. Missile
+10 to Energy
+10 to Vitality
+15% Damage Taken Goes To Mana
Magic Damage Reduced by 3
+33 to Mana
Damage Reduced by 7
+5 to Light Radius
Socketed (3)
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How to obtain Radiance

The fastest way to get this item on Battle.net is via trading, you can use the Item Price Checker to see how much Radiance is worth.
Another way of getting this Runeword is by farming the runes and the base. Find the drop rates of these items in the Item Database.

Radiance bases

The best base for Radiance is Grand Crown. Below is a list of all eligible 3 socket bases for Radiance.