Rex

For generations, aberrations had gone for periodic swims in the wyrmwound, absorbing it's toxic bacterial ooze into themselves to hasten their mutations. To make themselves stronger. But they had to be cautious about how often they indulged in these swims, lest they someday never emerge from the soup.

Rex, in his haste to become strong, threw caution to the wind, and did something no sane dragon even in plague flight would do- he used magic to merge the essence of the wyrmwound to himself, so that in a sense he would always be swimming in it. This went about as poorly as you can imagine, and the experiment nearly resulted in his demise.

But it just so happened that the witch Echo was traveling through plague territory looking for potion ingredients at the time of his collapse. She found him, barely alive and offered him a deal; she could make an antitoxin that would treat his affliction. However, it would only work for a day at a time, and required fresh ingredients so it couldn't be stockpiled. She could keep him alive, but he would have to stay near her hut for the rest of his life.

Desperate, Rex agreed, and to his shock the witch was as good as her word. The potion she gave him reduced his pain to a manageable level, and strengthened his body enough that he could move and follow her home. Deliriously grateful to his savior, he bears her no resentment for keeping him close, and willingly wanders the swamp to chase away would-be heroes that might try to harm her. Chronically ill and constantly in pain though he is, Rex did gain a considerable amount of strength from the mutations he inflicted upon himself, so he is not to be trifled with...