I don't think it would be HORRID for you to do that, but it is rather tough if they fail the check. Also, don't mummies get 3 attacks per round (claw, claw, bite)? That could potentially be 3 checks per round.
If you don't give them a way to possibly make it out if they get the rot, then the battle might be too difficult. If you do, however, give them a way to slow down the con loss or to cure themselves eventually, then it's no problem!
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