You are a Monstrosity

+2 Int -2 Str +2 Dex

Whisperkin mature at similar rate to humans, reaching physical maturity around age 50. They can live 150 to 300 years.

Whisperkin are between 6 in. and 2 feet tall and average about 25 pounds. Your size is Tiny.

Your base walking speed is 20 feet.

Accustomed to life underground, 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 can take the Disengage or Hide action as a bonus action on each of your turns.

Due to the strange origins of your race you gain proficiency in Stealth, or Arcana.

Soul Seed:
At 1st level, you know one cantrip of your choice from the Necromancy or Divination schools
At 3rd level, you can cast an additional spell from the Necromancy or Divination schools once per long rest.
At 5th level, you can cast an additional spell from the Necromancy or Divination schools once per long rest.

Veil of the Servant:
When a spellcaster casts find familiar, a nearby wandering (or stalking) Whisperkin using the Veil of the Servant a necromantic mimicry that cloaks their soul and form in a subservient resonance.  So instead of the familiar asked for it gets of the Whisperkin instead. The caster perceives it as a large enchanted mouse very unlike the creature shape it initially tried to summon. The form that the Whisperkin takes is its own and cannot be changed by recasting find familiar unless some other spell or disguise is used.

Much like a regular familiar if the caster is within 100 feet of the Whisperkin, they can communicate telepathically. Additionally, as an action, you can see through your familiar's eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has. During this time, you are deaf and blind with regard to your own senses.

Unknown to the caster the Whisperkin can also see and hear through the casters senses with the same loss of personal senses when in use.

Finally, while the caster can attempt a spell with a range of touch, through the Whisperkin. The creature can forgo use of the spell to make its own reaction. In this case the spell fails to cast without loss of the spell slot. The Whisperkin must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.

Divination or identify spells return information about Veil instead of the Whisperkin itself, making the detecting the fake familiar very difficult. 

Behavior:
The Whisperkin familiar acts loyal and subservient all the while fulfilling its own objectives under the familiar guise. The caster will find out very quickly that the Whisperkin will not put itself into danger like a regular familiar (since if it drops to negative 10 hit points it doesn't just get unsummoned but is instead very dead). Additionally it cannot be dismissed into a pocket dimension and resummoned much to the confusion of the caster.

You know Common and Whisperkin