Undead Familiar

Hold a small skull or skeleton of any Tiny creature atop a vial of blood and recite the incantation and the skull or skeleton will be imbued with false life for up to 24 hours. The caster will be linked to this skull or skeleton, animating it as a skeleton of the type of creature it is. For the next 24 hours, whoever's blood was used during the casting of the spell will feel a mental link open into the mind of used creature, allowing them to use the senses that the creature would have used while it was alive. As an action, the "donor" can choose to enter or leave it's mind to control its body or return to theirs. 

-Ex: Rat Skull - The eyes of the skull will be filled with a bloody red light and the "blood donor" will be able to access the senses that the rat's skull possesses.

-Ex: Bat Skeleton - The entire bat glows with a bloody red light and the caster can then use subsequent actions to control the body of the bat when they activate the mental link.

User remains immobile while they are seeing through the senses of their linked creature (EX: 1 mind can occupy a body at a time, your mind leaves your body to instantly enter the familiars, while it retains its own body). If the mental link is disrupted by any circumstances, the creature's body stops whatever it was doing until the "donor" reenables the mental link (airborne units will fall from sky unless given mental order to flap wings and has a safe mental disconnection). If "donor" takes damage from any source, the mental connection is immediately cut and any attack against the "donor" while mentally linked is a critical hit because they are unconscious because their mind is not in their body. If the familiar takes damage while the "donor" is linked to it then the "donor" takes 1d4 psychic damage plus 1/4 of the damage done to the familiar. 

The familiar is small but it can pick up items and manipulate objects within its power (EX: a rat cannot open a jar or door handle, but can carry a key or vial)

It's starting HP is determined by it's species, and it starts with half of that maximum, round down, minimum 1. Familiar can attack, but has disadvantage on the attack roll everything including damage is halved.

Donor will constantly feel the location of the familiar to within 2 meters, at a range of up to 1000m. If the familiar exits this range, the spell ends and the skeleton returns to death. 

Notes:

If a complete skeleton is not used, only the skull will be animated.

If the familiar remains undamaged, spell can be cast on it again as long as the correct type of blood is used. 

If animating a skull of the donor's species, the blood vial must be taken from a deceased individual (EX: Human Skull will need a dead human's blood to complete the ritual while a living human's blood will make them the Donor. Important if using blood from multiple humans)