Tieflings

Humanoids of tainted line, either their ancestors made an unholy devil pact that tainted their blood. Or an affair with a succubus a few generations back. Whatever the case, the fiendish blood marks the Tiefling.
Often sporting horns, strange skin colours from taints of red to blue and many others. Heavily dependent on their ancestry.

Pact born Tieflings

Most Tieflings of a devilish heritage are born of pacts, with one of the Nine Archdevils or their servants. Depending on the details of the Contract it can take several generations for one with a Tiefling to show up in a family. Or a family is cursed to only have every other generation by a Tiefling. Details of this are highly contract depended.

Cambion Lined Tieflings

Tieflings can also descend of any other fiend, through consummation. However, this always has the in-between status of the Cambion.
For example, a woman is impregnated by an incubus, then births a cambion. Followed by a cambion with another humanoid, results in a Succubus Tiefling.
In theory, this chain can contain any type of Fiend at the start. However, Incubi and Succubi are the most common.

Rakshasa Tieflings

Rakshasa Tieflings are born as the mixture of a Rakshasa, and another species. Following the traditional of crossbreeding outlined in A Bard's Guide to Crossbreeds. They somewhat resemble whatever appearance their sire had.