Devil dragon

How do I kill the devil dragon?

You need the Blessed Dragon Blade.

How do I get that?

You can get the Dragon Banger sword from Credne the Blacksmith but then you must get that sword blessed by The Lady of the Lake.

Or you can buy it from Vlasick the Smith but you might have to camp out outside his shop as it turns up in his inventory rarely but it does turn up if you're patient.

Where on the map are they located?

Credne's in his forge in the Ironstone Mines in the northeast. The Lady of the Lake is in the Lake of Reflection in the midwest. You should work with these to get the sword as otherwise you may have a problem.

The Lady of the Lake will require Mind Magic greater than 90 and an Emerald of Wisdom. Glenys in Merchants' Ford sells the emerald. You'll need the Kill the Devil Dragon quest to get the sword from Credne.

Alright thx for your help