Perhpas it was actually a lake of superheated caustic soda (sodium hydroxide) operating under extreme conditions. In reality, caustic soda is highly effective at breaking down organic material through a process called saponification, where it turns fats into soap and digests proteins into a slurry.
While this process would normally take hours or longer to fully dissolve a human body, there are ways to make it work much faster in this scenario.
If the lake were maintained at an unnaturally high temperature—say, just below boiling—it would dramatically accelerate the reaction, breaking down soft tissue within minutes rather than hours. Additionally, the presence of magical properties could further enhance the speed and efficiency of the process, allowing it to bypass natural limitations. For example, a magical catalyst could ensure that the reaction occurs uniformly across the body rather than being constrained by surface area or diffusion rates, enabling complete dissolution in seconds.
The caustic soda approach also has a certain gruesome plausibility: it would bubble, hiss, and emit noxious fumes as the victim's body rapidly saponifies, leaving behind only a greasy, gelatinous residue and dissolved bone fragments. The addition of magic ensures that the process reaches its horrifyingly rapid conclusion, making the scenario scientifically inspired but sufficiently fantastical for the story.
(Just thought I'd throw my two cents in,
@eaglejarl @Velorien @Paperclipped.)