Obsidian Serpent

[Azayaka]

Now, if you play Blade & Soul you've probably seen the Obsidian Serpent in the Wardrobe, this outfit is obtained by defeating the boss in Shadowstrike. It is a rare drop from the last boss, but it is how you obtain it, it's not dropped by any box or something like that, it's a drop from the boss just like the mask and the helmet.

[Kirvin]

Redacted.

[Azshura]

The last boss have a very limited loot table, sometimes she might not even drop a chest at all, which is most of the time. Just keep trying and she will eventually drop a chest, then pray the outfit is in it.

[Xaelon]

I have both the mask and the costume. What I have read about it on the web suggests it's a very rare drop.    I got very lucky with both drops.  Do note that the drops themselves come from the actual boss, not the box at the end. For 'optimal' farming you can let the adds that Nightshade summons at the end kill your escort target.  That way you can just restart the quest after killing her (rather than finishing it, abandoning it, exiting the instance, re-entering the instance, and taking the quest again).  If you let them kill him BEFORE Nightshade spawns you'll have to do the entire thing over again.

 

I assume you know this but if you complete the daily and then turn it in, you'll have to wait for the reset before you can enter the instance again.

[Zeykes]

Or if Dahua survives, just abandon the daily and pick it up again (involves a slight bit more running).