"A pustulent girdle that slowly undulates with latent vitality."
The skill description says it reduces poison damage by half, but it actually adds a 50% resistance, directly on top of other poison resistances you can get from from Shake it off and Poison tolerance in the Endurance skill tree. This means that if you have both of those skills, you can have complete poison immunity while wearing Slog's Annunclus. You can still get poisoned, but will receive no damage from it.
Its poison resistance even stacks with duplicates of this item, which allows you to gain complete poison immunity without the Endurance skills.
Note that while it is described as a girdle in its description, it is worn on your arm.