Sadras
Legend
The GM doesn't like how the PCs won, and so changes the outcome.
So a combat is falling short of being challenging and maybe even entertaining (drama element), which can easily happen when you're converting modules from one edition to the next. You decide at some point as DM to add an additional critter or two, increase opponent hit points, add or improvise a spell or ability...etc
The combat becomes enjoyable and no one is the wiser, except in this guy's case cause he lacked the necessary experience. Sure the outcome has been engineered, but @pemerton what does it matter if it is done in the moment, like your story now approach this is very much encounter now.
