Put everything into a saucepan over medium heat except the Swerve.
Cook stirring frequently until the cranberries pop. There will be more liquid than you will want initially, and will pour some off for another use later.
Remove from stove. In order to not crystalized, wait until the cranberries come down to 108 degrees F. before adding the erythritol. This will prevent that icing effect. Once you stir that in, allow to cool further, and then pour any excess liquid into a jar and use for drinks, etc.