Muldarion Treesearcher, Sage

Level 0 is not you're first level, level 1 is you're first level, level 0 isn't a level, the rules are you're first level, everyone's first level is 1, just like level 0 wizards don't get level 0 cantrips because you're not a wizard yet

I'm still not sure what you're trying to say here. Abyssal Tiefling's innate casting is phrased exactly the same as the other tiefling variants except it specifies a level. If he was playing one of those would he get the cantrip while at level 0?

Are you talking about infernal legacy

Yes, and whatever the different one from the SCAG is called(Silver tongue?).

I don't have the SCAG book so I can't see but the infernal legacy says you know instead of at level 1 or at first level

Yes, but I think the intent of Abyssal Arcana was that it's meant to work the same way, they just specified 1st level because they assumed that's when anyone would start a character.

That's why I take at first level as level 1, if you die as a teifling level 5 and get resurrected but say cursed as the abysmal teifling are you a level one or a level 5, if we started at level 3 would the person be a level 3 abysmal teifling or a level one since that's the 1st level, the vagueness of the at first level can be interpreted as anything