Looks like a burn to me

1)Go to a vet IMMEDIATELY! They will prescribe antibiotic cream.
2) get some betadine and clean the wound in the mean time
3) change substrate to paper towel.
4) check your husbandry and set up.

Is your heat pad hooked up to a thermostat? The thermostat is needed to regulate the heat source. Otherwise an unregulated pad can hit 115-130 degrees, which is enough to burn a snake.

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