A Tiefling has an innate amount of devil inside of them, using this feat they tap into their ancestors' genes. After being knocked unconscious they roll a d20, if they succeed on the DC, the Tiefling goes under physical transformation, their horns grow, hair ignites, eyes have flames in the irises, and their muscles increase in mass. Tieflings are fairly different from said devils, although this transformation heals them, it puts a heavy strain on the body. The body splits and tears open, leaving lacerations and cuts, causing fairly heavy bleeding
Tiefling undergoes a temporary devilish transformation, gaining advantage on melee attack rolls, +3 to melee damage rolls, and a +4 to STR score. They also suffer 2 points of bleeding damage (does not damage temp health) on each subsequent turn until healed or unconscious.