Half-Fiend Tricksters have slightly smaller builds than their mortal kin. Your size is Medium.

Half-Fiends mature at a slightly slower rate to their mortal half, but their lifespans often last closer to 1000 years.

Half-Fiends are typically inclined towards chaos, but not necessarily evil. Many even rebuke their demonic heritage and walk the path of good.

You can speak, read, and write common, abyssal, and infernal.

You gain a +2 to dexterity and a +1 to intelligence.

You have resistance against poison and acid damage but vulnerability to fire and radiant damage.

You have dark vision out to 60 ft.

Once per long rest, as an action, you can transform into your fiendish form. This transformation lasts 1 hour, until you revert forms, or you become incapacitated, whichever comes first. During each long rest you roll a d6, if you roll a 1 you dont gain the benefits of a long rest or the ability to transform again.

Fiendish Form

Medium fiend (devil)

Armor Class 16 + your dex. Mod. (Natural armor)
Hit Points (1d10 + your con. mod.) × your level
Speed 40 ft., fly 30 ft.

Damage Immunities poison
Condition Immunities poisoned
Senses darkvision 120 ft., passive Perception 18
Languages same languages you spoke before + 120 ft. of telepathy

Adaptive Skin. Your skin is able to adapt to your surroundings giving you advantage on stealth rolls
Devil's Sight. Magical darkness doesn't impede the devil's darkvision.

Limited Spines. The devil has twelve tail spines. Used spines regrow when you transform again.

Flyby. If you used the dash action (or bonus action) you don't provoke opportunity attacks while flying.

Actions

Multiattack. The devil makes three attacks, divided among the attacks you can make. 

Claws. Melee Weapon Attack: +6 + your proficiency bonus to hit, reach 5 ft., one target. Hit: 8 (1d4 + 6) slashing damage. 

Tail Spine. Ranged Weapon Attack: +6 to hit + your proficiency bonus, range 30/120 ft., one target. Hit: 8 (1d4 + 6) piercing damage.

Stinger. Melee Weapon Attack: +6 + your proficiency bonus to hit, reach 5 ft., one target. Hit: 8 (1d4 + 6) piercing damage, and the target must make on a DC 12 Constitution saving throw, taking 2 (1d4) poison damage and becomes poisoned for one minute on a failed save, or half as much damage and no poisoning on a successful one. It can repeat this save at the end of each of its turns, ending the effect early. 

Equipment. You can also equip weapons, replacing your claw attack. If you replace them with a 2-handed weapon you can only make 1 attack with it but still gain your other 2 attacks.