It's pretty simple.
Open your Object editor (OE), you can also open it by pressing F6.
Right click on a folder on the left side and choose "New Custom Unit"
You can delete his current abilities under "Abilities - Hero"
Next go to the "Abilities" tab (Also in OE)
Right click on a folder on the left side and choose "New Custom Ability"
Find Rain of Fire and name it however you want.
Go back to your Dreadlord and open "Abilities - Hero", and add your new custom ability. (You can filter custom abilities).
I attached a simple map so you can open it and look for yourself and test it.
You should check out tutorials about World Editor
And really many more places, just search it up. There's a whole tutorial section here