The evil lord Darth Vader, obsessed…
‘lord’ here isn’t being used as a title but rather a descriptor. He is a ‘lord,’ and is called Darth Vader.
It’s the same as if you were to say:
The valiant knight Sir Arthur…
edit: if it were being used as a title, it would be capitalized.
edit: also crucial here is the definite article “The.”
“Lord Vader” vs the “THE evil lord Darth Vader”