Cathan's Rule is a Set Battle Staff that is a part of the Cathan's Traps in Diablo 2 Resurrected (D2R). This normal Staff has a quality level of 15 and a treasure class of 18. This is the only Set item in the game that has +X to Fire Skills attribute.
The fastest way to get this item on Battle.net is via trading, you can use the Item Price Checker to see how much Cathan's Rule is worth.
Farming it by killing monsters is possible, check the drop rate table below for more information.
It can also be gambled but the chances are very slim.
Cathan's Rule drop rate table
The monster with the highest drop chance of dropping Cathan's Rule is The Cow King on Nightmare difficulty in The Secret Cow Level of Act 1. Farming the monsters listed below gives you the highest chance of finding that item.
Cathan's Rule variants
Below is a list with different variants of Cathan's Rule. See the stats of its normal or upgraded variant.