Black Light Burns is a main quest available in Rosaria of Final Fantasy XVI.
Your objective is to find a way into Port Isolde via an alternative path.
Here’s a walkthrough of Black Light Burns in Final Fantasy 16.
Find an entrance to the Lazarus District
Head to the quest marker shown on the map. It’s the entrance to the Lazarus District which is a shortcut to Port Isolde.
Defeat the bandits
Enter the compound and you’ll be ambushed not long after.
Defeat the bandits.
During the cutscene, you’ll realize that they belong to the Guardian of the Flames and is led by Sir Wade who was thought to be dead 18 years ago.
Speak with Wade
After the cutscene, speak with Wade.
You’ll now be told that Clive can be joined by various companions but the companions may stay behind depending on the destination.
Black or White sub-quest
You will now start a sub-quest titled Black or White.
Make for Bewit Bridge as marked in the map below.
When you reach the area, you’ll see that the Black Shields have been waiting. Slay them all.
More enemies will come including a wyvern.
Defeat the Knight of the Lasting Dark
A dragoon called Knight of the Lasting Dark will show up.
Slay the dragoon. The battle style is similar to the other dragoons from before.
This sub-quest ends here.
After the fight, you’ll be back at Lazarus District. Wade will give Clive the Burnt Blade seal.
It unlocks side quests for the area.
Later, Clive and Jill manages to meet Byron Rosfield. The latter is finally convinced that Clive is still alive and decides to help procure a ship.
Return to the hideaway
After the cutscenes, head back to the hideaway.
The quest ends after you reach the hideaway.