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I have a lot of story planned and it's just as lame for me as it is for them. I accidently threw a couple Mud Lashers that were too difficult after they had defeated the huge Mudbeast. This was honestly just a filler quest to waste their time but it just killed them. Sadly we're running a two person thieves guild campaign without a cleric so it's fairly difficult to scale for them so I feel like I'm to blame just as much as they are for not running away at low health.
I've thought of some solutions like removal of enchantments or skill penalties but they don't feel good enough. I was wondering what other DMs would do - how should I handle this situation?