This Witcher 3 side quest is ripe for either an investigative group a combat-heavy group, as the party will need to plan a launch point and then assault an island infested with undead monsters. There's tons of room here to tease the story as each combat encounter is followed by clues as to what happened with the recently-deceased locals.
The means of telling the story could be switched up to be uncovered through various investigative abilities and wizard / cleric spells, or you could keep it as-is with ghosts re-enacting past events if you want to run this as a spooky Halloween one shot.
If you've got any aspects of plague or evil wizards running amok in your campaign, A Towerful of Mice is also simple to plop into the existing storyline.
Recreating the interior of the tower at the center of the island itself as a dungeon to be stomped includes some interesting possibilities for D&D. Rogue abilities for detecting traps and climbing, as well as low level wizard spells like Mage Hand, can be very useful.
If you want to connect this quest to a larger campaign, introduce characters to take the parts of Annabelle, Alexander, Kiera, and Graham ahead of time. If you're already running a campaign, there's no reason those roles couldn't transition to existing characters your party knows well for a bigger impact.