[ravagernl]

First what you would need to do is check if the given unit by UNIT_AURA is 'player'. This event runs pretty often, and for each unit.

local unit = ... -- select the first argument from arguments passed
if unit ~= 'player' then return end -- run code only for player unit

Using return will skip out of the function stopping any processing of code. It's just a way of avoiding too many nested if then end's.

Then you know it's the player that gained or lost a buff. You want to see if the player has gained righteous fury. To see that, we use UnitBuff:

local active = UnitBuff('player', 'Righteous Fury')
if active then
    print('Righteous Fury has been activated')
end

or even

if UnitBuff('player', 'Righteous Fury') then
    print('Righteous Fury has been activated')
end

But what will happen, is that will fire off every time you get or loose an aura, resulting in chat spam. So we solve that with a variable called 'hasrf':

local active = UnitBuff('player', 'Righteous Fury')
if not hasrf and active then -- we gained righteous fury as we did not have it before and it's active now
    hasrf = true
    print('Righteous Fury has been activated')
elseif hasrf and not active then -- we lost righteous fury as we did have it before and it's no longer active now
    hasrf = false
end

Be sure to define 'hasrf' as a local variable outside your eventhandler function