Vulnerability to Baator

Idea; Magic

Vulnerability to Baator is one name for the unbreakable link a tiefling has to the devil connected with their family.

Disclaimer

While she was alive, Uhoshi Nevro researched the possible implications of her link to Mephistopheles. Reliable information is hard to come by, since the aristocrats of Baator routinely destroy both books and sages with merciless diligence. Still, the Order of Silver Comets has ways of keeping secrets and secret-keepers safe. The following information is probably true.

About the Vulnerability

All tieflings are vulnerable to infernal manipulation, in theory. This is inherent to their "blood" (i.e., their DNA). However, tieflings are mortals with souls. Thus they are supposed to have free will. A devil can't directly compromise free will (or Asmodeus and his Ruby Rod will destroy them). They can still tempt a tiefling into using the power their inherited: innate spellcasting. (Using darkvision or Hellish Resistance doesn't seem to count.) The more a tiefling relies on their Infernal Legacy, the more connected they become to their progenitor, thus more vulnerable to manipulation by them or their agents.

Even then, intention matters. A tiefling can balance their Vulnerability to Baator through commitment to its opposites: kindness, generosity, and especially selflessness. In other words, a tiefling can benefit in a distinct way from following the Path of Giving. For example, suppose a tiefling of Mephistopheles uses their Flame Blade to rob a merchant. That would make them more vulnerable than using it to battle an evil necromancer.

Tieflings who are Unbound are further able to balance their Vulnerability to Baator with their own strong will. Similarly, tieflings can balance their Vulnerability through devotion to a powerful patron (e.g., a god, a Fae aristocrat, a djinn).