A particular species of merfolk inhabit the abyss of the pacific ocean, they belonged to the secret clan which was tasked by the Atlantean emperor to seek immortality elixir. The emperor ordered their vocal cord to be removed as a punishment for not able to find the elixir and they are exiled into the abyss, without echolocation they must rely on vision which is similar to humans. I am wondering what can the merfolk use to lit up the abyss without invention of incandescent lightbulb, time period is early 14th century AD?
Anglerfish come to mind.
There is evidence that giant squid are bioluminescent and that they use flashes to stun and catch prey. You may also consider for a really heavy metal factor, specially pirate metal. Suggested theme song: Leviathan, from Alestorm.