This race is genetically designed with the intent of creating evil infiltration units, although with some mistakes.  They share some of the heritage of both their Tiefling and Drow origins, but there is reason to believe that, in appearance, some look very much like Elves and some very much like Tieflings.

The Tiefling infernal heritage, as much as was considered reasonable, was left intact in all of the Hybrids.  However, their appearance was certainly tampered with. Tieflings have large horns that take any of a variety of shapes and this was retained in the Tiefling-resembling Hybrid design.  The notes indicate that their trait for tails was removed, as was their trait of canine teeth.  The Hybrid has eyes that are uniformly dark solid colors—black, red, any very dark color—with no visible sclera or pupil. Their skin tones are dark, but also include various shades of red. Their hair is also uniformly dark, from black or brown to dark red, dark blue, or purple.  It is likely that these Hybrids could easily be taken for genuine Tieflings by those not familiar with Tieflings.

Drow are descended from Elves and have their general appearance.  Some of the Hybrids were created with virtually no trace of their Tiefling heritage except for a darkening of the skin and eye color similar to that found in their Hybrid Tiefling counterparts.  They are slender and graceful, with the pointed ears, eye shape and general characteristics of their Elvish cellular structure.  They also stand somewhat smaller than their Tiefling-resembling counterparts, although still well within the range of normal height and weight for a human.  Passing as an Elf, especially to any not familiar with those of Elvish descent, would be very easy for this strain of Hybrid.

We have no idea how the Drow/Tiefling Hybrid will react to the outside world.  However, it would be safe to assume that the Hybrid would be very suspicious, afraid to trust anyone and definitely wary of, or even frightened by, anything that resembles high technology or scientific apparatus. 

The beings (and their race) responsible for the creation of this Hybrid race, and the subsequent attempted destruction of them, is completely unknown.  There is a very high probability that the Drow/Tiefling Hybrid would attack any member of that race on sight.

There has been no record of anyone encountering this Hybrid so far.  It is possible that the Hybrid has simply died.  This is considered highly unlikely and it is more probable that the Hybrid has blended into general society.  They were designed and trained to be self-sufficient, resilient, opportunistic as well as friendly and personable (for infiltration purposes).  If more than one did escape, they may have even formed their own enclave or settlement somewhere.

It is apparent from the notes that several major abilities and skills were missing from the Drow/Tiefling Hybrid, whether by accident or by deliberate design.

The ability of Elves to trance instead of sleep is totally absent from the Hybrid, as is the immunity to magically induced sleep.  The Fey Ancestry advantages and skills are totally missing as well, although proficiency in other skills was somehow added.

The abilities of the basic Tiefling appear to have been retained more or less intact regardless of appearance.

It is not known what names these Hybrids will take.  They were referred to by batch number and individual number within that batch when they were in the laboratory.  It is extremely unlikely they will retain these identifiers, and will instead simply adopt any name that takes their fancy.  It could be one they have heard, one they read about or even one of the names of their designers.  

As with so much other information about this race, the information here is fragmentary.  The notes that were found seemed to indicate that these Hybrids were supposed to be completely evil.  However, other notes hinted that the evil alignment, supposedly bred into these creatures, had gone horribly wrong, and various individuals were demonstrating what was referred to as "aberrant alignments". 

One thing was clear.  Individuals of both Lawful Good and Chaotic Evil alignments (as well as any other alignment combinations) existed.  This "flaw" in the Hybrid design was eventually one of the reasons for the apparent destruction of the race and the laboratory in which they were created and kept.

At this time, the alignment of the surviving Hybrid (or Hybrids) is not known.

As a result of your combined heritage, both your Charisma and your Dexterity increase by 2.

The life span of a Drow/Tiefling Hybrid is unknown. guessing 300 years
A Drow/Tiefling Hybrid (of either appearance) is roughly the same size and has the same variations as a human.  Your size is Medium.
Your base walking speed is 30 feet.

The scientists managed to instill certain skills that their creations knew "instinctively", although it is unknown if this was by accident or design.  You have Proficiency in both the Stealth and Deception skills.

The genetic designers retained your Drow vision.  You have darkvision with a radius of 120 feet

 both magical and nonmagical 

You know the Thaumaturgy cantrip. When you reach 3rd level, you can cast the Hellish Rebuke spell (as a 1st-level spell) and the Disguise Self spell with this trait. Charisma is your spellcasting ability for these spells.

You have proficiency with rapiers, shortswords and light crossbows.

be resistant to stuff

you know languages tbd

You can use your action to exhale destructive energy. Your draconic ancestry determines the size, shape, and damage type of the exhalation. When you use your breath weapon, each creature in the area of the exhalation must make a saving throw, the type of which is determined by your draconic ancestry. The DC for this saving throw equals 8 + your Constitution modifier + your proficiency bonus. A creature takes 2d6 damage on a failed save, and half as much damage on a successful one. The damage increases to 3d6 at 6th level, 4d6 at 11th level, and 5d6 at 16th level. After you use your breath weapon, you can’t use it again until you complete a short or long rest.

magic 2