Blood Fiend

The caster cuts their arm taking 1d6 of slashing damage (and the caster can not regain those hit points until the duration ends, caster loses concentration, or the spell is ended intentionally) as the blood starts to come out, it starts to take shape into a fiendish creature. Depending on the level it was cast the form is more skeletal or not. The caster is in complete control of this creation of necromantic energy to the point of the Blood Fiend repeating every word the caster says. When it isn't making screams and gurgles. The Blood Fiend can turn into a pool and hide making a sneak attack. When the spells duration ends naturally the blood fiend goes back to the casters arm and re-enters the bloodstream (the caster gets their hit points back and if damage was taken the caster gets the remaining hit points back). The blood that  If the caster loses concentration during the duration of the spell the Blood Fiend loses its shape and turns into a pool of blood. (The Blood Fiend has a AC of 12 due to the fact that it can revert its form into a pool of blood.)