Casting the spell summons a demon after 1 hour, deals damage equal to half your max hp, reduces you max hp to 3/4 the original value.

Once the demon appears it will consume the animal, then you must offer the vial of blood then the heart to the demon. The demon will drink the blood an absorb the heart. If the blood is from one of your allies, you gains control over the demon. If the blood is from someone neutral to you, you gains the demon's protection. If the blood is from someone that is hostile towards you, you gains its enmity.

The demon gains the form of the creature that the heart came from, and their stats

If you gain control over the demon:

[The demon has chaotic evil alignment and acts independently unless given a command. In combat, it rolls its own initiative and acts on its own turn.

When the demon drops to 0 hit points, it explodes, all creatures within 20 feet must succeed on a dexterity or constitution saving throw or take 1d6 necrotic + 1d6 fire damage.

You can summon your demon to anywhere within 30 feet of you if it strays over 100 feet away, in addition it must carry out any commands you issue it.

As an action, you can temporarily dismiss the demon. It disappears into a hellish pocket dimension where it can leech life off any creatures it finds. Alternatively, you can dismiss it forever. As an action while it is temporarily dismissed, you can cause it to reappear in any unoccupied space within 30 feet of you. This pocket dimension is the only dimension where the demon can be summoned from, if it is in a different dimension from you it gains its freedom.

You can't have more than one familiar at a time. If you cast this spell while you already have a familiar, the demon consumes it.

Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 50 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 the demon's modifier for the roll.]

If you gain the demon's protection:

[The demon must follow you and protect you during combat, you cannot command it, the demon can talk to you telepathically at any time but you cannot talk back, the demon will perish if you fall below 0 hp, instantly bringing you up to full health. The demon has a chaotic evil alignment and will often cause chaos, trouble, and misfortune in your wake. It gains +2 to every stat

When the demon drops to 0 hit points, it explodes, all creatures within 20 feet must succeed on a dexterity or constitution saving throw or take 1d6 necrotic + 1d6 fire damage.]

If you gain the demon's enmity:

[The demon will immediately try to kill you, and succeed on the initiative roll, If you manage to defeat the demon you can cast the spell again using the demon's heart, blood, and corpse, the demon will reappear absorb its body and you gain it's gratitude and control over it, and the demon gains +5 to all stats.

When the demon drops to 0 hit points, it explodes, all creatures within 20 feet must succeed on a dexterity or constitution saving throw or take 1d6 necrotic + 1d6 fire damage.

he demon has 15 +1 per player level, It's the same level as the player. The demon knows Prestidigitation, Unseen Servant, Blindness/Deafness, Darkness, Invisibility, Fly, Gaseous Form, Vampiric Touch, Burning Hands, Eldritch Blast and Hellish Rebuke. The demon learns more at higher levels, 3, 5, 15, 18, 20. [3]Fireball, False Life. [5] Dimension Door, Fire Shield, Wall of Fire, Conjure Elemental, Hallow, Flame Strike, [15] Circle of Death, Finger of Death. [18] Power Word Stun. [20] True Polymorph. ]

If you gain the demon's gratitude, decided by the DM or by killing and re-summoning

[When the demon drops to 0 hit points, it explodes, all creatures within 20 feet must succeed on a dexterity or constitution saving throw or take 1d6 necrotic + 1d6 fire damage.

You can summon the demon to anywhere within 30 feet of you if it strays over 100 feet away.

As an action, you can temporarily dismiss the demon. It disappears into a hellish pocket dimension where it can leech life off any creatures it finds. Alternatively, you can dismiss it forever. As an action while it is temporarily dismissed, you can cause it to reappear in any unoccupied space within 30 feet of you. This pocket dimension is the only dimension where the demon can be summoned from.

You can't have more than one familiar at a time. If you have the demon's gratitude it can still follow you as a free creature, but you cannot issue commands or gain any benefits from the demon

Finally, when you cast a spell with a range of touch, the demon can deliver the spell as if it had cast the spell. The demon must be within 50 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 the demon's modifier for the roll.]