Although you are still a living creature, your flesh is infused with necromantic energies that make you, in some ways, akin to the undead.

You gain the following benefits:

• Increase your Constitution score by 1, to a maximum
of 20.
• You have resistance to necrotic and poison damage.
• You have advantage on saving throws against effects
that would give you the poisoned condition.

Sandy Petersen’s Cthulhu Mythos