The primary devil was Asmodeus, but all of the Eight took part. A few Dragon mag articles introduced at least two different bloodlines (tied to Mephistopheles and Baalzebul, IIRC), and the Player's Handbook: Tieflings also brough a few bits of lore about this stuff.Tim Baker wrote: ↑Thu Sep 27, 2018 11:08 pmIn Nentir Vale, there was a single civilization that decided to make pacts with devils to gain power against their enemies. I'm not sure if this was a single devil or multiple, but even if it was the latter, perhaps they were all under the same arch devil's hierarchy. So it makes a certain amount of sense that they would all look similar.
Big Mac wrote: ↑Thu Sep 27, 2018 9:13 pmI'm not sure if the same would apply to Forgotten Realms. Forgotten Realms had connections with the Great Wheel back in the 1st and 2nd Edition Eras and they rebooted the cosmology for the 3rd and 4th Edition Eras. And I'm not sure if the 4e origin of Realms tieflings is supposed to be tied in with Toril or Abeir.
Erin Evans' Brimstone Angels series. In short, a coven of warlocks knows as the Toril Thirteen tainted almost all tiefling bloodlines in Toril with a curse created by Asmodeus to change all tieflings of these bloodlines that born after that point in the planet to be "Asmodean" tieflings (so, 4e tieflings), so Asmodeus could ascent to godhood as a racial god. As Tim said, the curse was not perfect and the plan failed in the end (and Asmodeus became a god in the Realms by other means in the end, as well).Tim Baker wrote: ↑Thu Sep 27, 2018 11:08 pmI believe there was a novel that explained the change to tieflings' appearances in the Realms. Asmodeus was able to exert influence over all tiefling bloodlines, which made the tieflings appear similar to him. In 5e, the explanation is that this is still the most common appearance, but Asmodeus wasn't successful in converting all tieflings, so those who look different are the small minority that escaped that fate.
The curse was broken by the end of 4e, so new tieflings of other variants can born now in Toril, and obviously, the curse didn't affected the tieflings born in other planes and worlds. But, Asmodean tieflings still exist as a the most common bloodline in the Realms because of 100+ years of existing, making them common enough in other planes if you add to them the Asmodean tieflings native to Nentir Vale.