Your soul is your source of power, without access to it you are unable to cast spells. If you are separated from it in any way you temporarily lose all of your spell slots and sorcery points until it is returned to your body.

As your soul was once possesed by a powerful spellcaster, you gain access to spells from a different class as you level up. The class these spells come from will be chosen when you create your character and should be reflected in your backstory.

Arcane Inheritance

At level 1, choose a class that can gain the "Spellcasting" feature and that you have the prerequisites to multiclass, besides Sorcerer. At levels 2, 4, 6, 10, 14 and 18, gain a spell for which you have spellslots to cast from the spell list of the class you chose at level 1. It doesn't count towards your total known spells, and you can cast it using Sorcerer spellslots and you may use Metamagic with it.

Artificer

You gain access to Artificer spells that don't count towards your known spells.

Bard

You gain access to Bard spells that don't count towards your known spells.

Cleric

You gain access to Cleric spells that don't count towards your known spells.

Druid

You gain access to Druid spells that don't count towards your known spells.

Paladin

You gain access to Paladin spells that don't count towards your known spells.

Ranger

You gain access to Ranger spells that don't count towards your known spells.

Wizard

You gain access to Wizard spells that don't count towards your known spells.

Interweaved Experience

Once per long rest, you may expend up to 2 Sorcery Points to temporarily increase your proficiency bonus for 1 proficiency you have by the amount of Sorcery Points expended. This increase lasts 1 hour.

Soulbound Memory

You may expend any number of Sorcery Points to cast a spell you don't currently have access to from the class you chose at level 1 whose level is equal to the number of spell slots spent. The spell cast is cast at its base level and cannot be affected by Metamagic.

You must have seen this spell performed or you must have read about it to perform it. You should keep a list of spells you've seen cast from whichever class you chose.

Soul Tether

If you are ever reduced to 0 hit points or lower, you can instead use a level 5 or higher spell slot and all your remaining Sorcery Points to be reduced to 1 hit point and cast "Etherealness" on yourself. If you use this feature, you cannot use it again until you've completed 2 long rests.

You do not need to know "Etherealness" to use this feature, and you cannot cast it otherwise, unless you learn it.

Reincarnation

Once, when you die, you may cast "Reincarnate" on yourself. You cannot use this feature if you have already had the "Reincarnate" spell cast on you.

Your new physical form has -5 to strength, dexterity, and constitution for the month after it is reincarnated.

After this feature is used, choose 2 of the following effects:

1) If your character is bloodied at any time, they are frightened by whatever caused it to be bloodied. This effect ends when the source of fear is out of your line of sight.

2) At the start of each day, you must make a Constitution saving throw. If you succeed, nothing happens. If you fail, your exhaustion level becomes 2 if it was previously lower than 2. It cannot be lowered past 2, but it can increase. This effect lasts 1 day.

3) Choose 1; Your character is afflicted with indefinite madness and cannot be cured of it, or your character has amnesia.

4) Choose 1; When others attempt to intimidate you, you must make a wisdom save: if you fail, you are stunned until the source of intimidation is out of your line of sight. Or, your character is immune to intimidation and has disadvantage on all of their Social rolls.