Pre-made adventures give GMs a ready-to-go group of game sessions. They give you everything you need and you can have a group go through them over the course of several sessions. The Mermaid’s Blessing is a new adventure for 5th Edition that’s up on Kickstarter now. I don’t really think that these mermaids are going to be combing their hair with forks and singing with crabs.
From the campaign:
The Mermaid’s Blessing is a 5th Edition adventure designed to challenge experienced players. The environments and obstacles that the players encounter will test their creativity as well as their imaginations in more ways than just the CR of their opponents. Choices have consequences, and the players will face decisions that will not only put their lives on the line but the lives of innocents.
The party is summoned to the impoverished island nation of Zaltara by a friend desperate for their help. Unwittingly their friend has unleashed a wave of evil onto the island that threatens to kill and enslave all its inhabitants. Will the party succeed in saving the lives of not just their friend but everyone else on the island? Will they uncover the secret behind the invasion from the sea and defeat it, or will they unwittingly unleash an even deadlier threat that has lain dormant for a millennium?
The campaign is about 1/2 of the way to its goal with still 26 days on the clock.