Your abilities all increase by 1, except for your Charisma score which increases by 2.

Half-Tieflings, like their Human parent, reach adulthood in their late teens and live less than a century. While there are some who have been known to live around 110 years old, these appear to be exceptions based on a more concentrated Infernal Bloodline (i.e.: offspring of a Half-Tiefling and Tiefling). 

While Tieflings are not inherently evil, Half-Tieflings struggle with the chaos in their minds due to their mixed parentage. There are exceptions, but most Half-Tieflings have an independent nature that inclines many tieflings toward a chaotic or evil alignment. 

Like both of their parents, Half-Tieflings are medium.

Your base walking speed is 30 feet.

Thanks to your infernal heritage, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

You know the thaumaturgy cantrip. When you reach 3rd level, you can cast the hellish rebuke spell as a 2nd-level spell once with this trait and regain the ability to do so when you finish a long rest. When you reach 5th level, you can cast the darkness spell once with this trait and regain the ability to do so when you finish a long rest. Charisma is your spellcasting ability for these spells. Tapping into your Infernal Bloodline for these spells however, there is a chance that you will decend further into madness as your Human side tries to compete with the Chaotic Bonds of your Tiefling side. 

Using your Infernal Heritage to cast either hellish rebuke or darkness will result in a chance that you will decend into a madness called Chaotic Bonds.

Once the spell is cast, you must roll one D6. If the number is 3 or below, you are unaffected. For rolls 4 and above, you become more chaotic, perhaps jumping the gun in an non-combative situation or caring less about the consequences of your actions. Your physical Tiefling features also become more pronouced (e.g. your canine teeth may get sharper, your horns and tail may get larger, your eyes may be consumed by red or black or a reddish tint may start to appear on your skin) these features can be decided by the player or the DM.

The effects of Chaotic Bonds can stack, making you more unstable as it wears on; but can be alliveated by meditating and reconnecting with your human side during a long rest. At the completion of the long rest, the effect is removed but only if you spend some time focusing on it. Some find it helpful to carry a token of their humanity such as a small trinket from their family or friends for these purposes.

In combat, a party member may take an action to attempt to remind you of your Humanity and remove the effects of Chaotic Bonds if it starts to present issues. However it can be quite difficult and is calculated by a charisma saving throw contest between the party member and yourself. If successful, the effect is removed at the end of their turn, however, you are stunned for your next turn as you take time to come back to reality. If unsucessful, the effects remain. Advantage may be imposed on the party member at the DM's discretion depending on how they try to help (i.e.: a very close friend or someone who can recall fond memories of your travels may impose advantage).

If Chaotic Bonds is not removed in 24 hours, some of the physical changes may become permanent (e.g.: your horns or tail may now remain prominent or parts of your skin will remain a more Tiefling colour). This is at the discretion of the DM.

As a cantrip, thaumaturgy does not effect Chaotic Bonds. 

You can speak, read, and write Common, Infernal and one extra language of your choice.