I have a question for you all, because my Planescape lore is failing me. Sigh. This is what I get for not running a Planescape game for several years now, and only writing one Planescape themed piece (a Demonomicon articlefor WotC) since 2007. I get into other things and my lore gets hazy.
Are there any Planescape sources that specifically call out a tiefling being of elven heritage for the mortal fraction of their blood? I could have sworn that there was, but it might have been an aasimar.
I don't recall any elven tieflings in specifically Planescape sources, but Forgotten Realms sources include the Fey'ri and draegloth (and both half quarter-fiendish variations). There are also half-yochlols and Graz'zt has a half-drow son first introduced in the Shackled City adventure path.