In a skillet, cook coriander seeds over medium heat, stirring frequently, until fragrant. Transfer to a spice grinder, and pulse until most seeds are broken up.
In a large stockpot, combine seeds and cider. Bring to a boil over medium-high heat; reduce heat, and simmer for 30 minutes. (Be sure to measure the cider after simmering to replace any lost water content, as the sugar in apple cider can easily become over-concentrated.)
Stir sugar into cider mixture, and let cool completely. Refrigerate in airtight containers for up to 1 month, or freeze for up to 6 months.
Recipe by Louisiana Cookin' at https://louisianacookin.com/the-perennial-cooler/