At some point in your family tree, some kind of bog creature left its mark, which manifested in you at a young age.
A Green Hag might have charmed one of your ancestors, or a curse might have been put upon your lineage. Be it as it may, you have now manifested the outcome of this "infection".

Hag's bosom

At level 1 when you select this origin you gain the following traits:

• You gain resistance to Poison damage and advantage on saving throws against poison effects and the Poisoned condition;
• You are continually surrounded by a fine and hardly visible mist, making plants tend to wither and die near you.
  When casting a spell on your turn, you can make all hostile creatures within 5 feet of you Poisoned on a failed CON saving throw (normal spell DC) until the start of your next turn.
  When the spell triggering this effect was modified by any Metamagic feature, you can instead decide to make all hostile creatures within 5 feet of you Stunned on a failed CON saving throw (normal spell DC) until the start of your next turn.

Rotten substitution

At level 6 you learn how to better use your miasmas and weave them into your spells.
You add your Charisma modifier to all your spells normally dealing Poison or Necrotic damage.
Additionally, when casting a damaging spell, you can decide to spend 1 Sorcery Point to change the damage type to Poison or Necrotic. If you decide to change the damage from Poison to Necrotic or vice-versa you still add your Charisma modifier to the damage.

Putrescence

Your control of the corrupting energies flowing through your veins is such that you are now able to make your miasmas affect a distant creature. At 14th level, you gain immunity to Poison damage.
As a Bonus action in your turn spending 1 Sorcery Point, you can designate a creature you can see within 60 feet of you to be afflicted by the putrescence running in your bloodline. The target suffers Poison or Necrotic damage equal to you Sorcerer level+your Charisma modifier+your proficiency bonus on a failed Constitution saving throw (normal Spell Save DC), or half as much on a successful save, each turn for a number of turns equal to your Charisma modifier.
You can use this feature once every short rest and on only one target a time.

Death and Decay

You release the full horror hidden in your blood, releasing a miasma that corrupts and withers everything it comes into contact.
At 18th level, you learn the Cloudkill spell, if you do not already know it, and it does not count for the total of your known spells.
Additionally, as an action on your turn, you can cast Cloudkill as if you cast it at 8th level and centered on yourself. The cloud moves with you instead of on its own, and the spell automatically benefits from your Rotten Substitution feature. When casting the spell this way, you can spend 1 Sorcery Point and choose a number of creatures up to your Charisma modifier. The chosen creatures automatically succeed on their saving throws against the spell, and they take no damage instead of half damage. You can use this feature once, and regain the ability to do so after a long rest.