Rain is a Runeword "OrtMalIth" in Diablo 2 Resurrected (D2R). It requires these 3 runes to be inserted into a socketed item in the exact order: Ort + Mal + Ith . The eligible bases for Rain are the following: Body Armors . This is the only Runeword in the game that has X% Chance to cast Level X Cyclone Armor when struck attribute.
RainRainDusk Shroud
Ort · Mal · Ith
Defense: 361 - 467
Durability: 20 of 20
Required Strength: 77
Required Level: 49
5% Chance to cast Level 15 Cyclone Armor when struck
5% Chance to cast Level 15 Twister on striking
+2 to Druid Skills
+(100 - 150) to Mana
Lightning Resist +30%
Magic Damage Reduced by 7
+15% Damage Taken Goes To Mana
Socketed (3)
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How to obtain Rain

The fastest way to get this item on Battle.net is via trading, you can use the Item Price Checker to see how much Rain 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.

Rain bases

The best base for Rain is Dusk Shroud. Below is a list of all eligible 3 socket bases for Rain.