Combine 0.5 gallons of water, kosher salt, brown sugar, peppercorns, garlic, rosemary, thyme, bay leaves, and allspice in a large stockpot.

Place the stockpot over medium-high heat and bring to a simmer, stirring occasionally until the salt and sugar are completely dissolved.

Remove the pot from the heat and stir in the 8 cups of ice and the remaining 1.5 gallons of cold water to rapidly lower the temperature.

Ensure the brine temperature has dropped below 40°F (4°C) using a probe thermometer before proceeding.

Place a thawed, cleaned turkey (neck and giblets removed) into a food-safe brining bag or a non-reactive 20-quart container.

Pour the chilled brine over the turkey until it is fully submerged; weight the bird down with a heavy plate if necessary.

Seal the bag or cover the container and refrigerate at 38°F (3°C) for a duration of 12 to 24 hours.

Remove the turkey from the brine and discard the liquid. Do not reuse the brine.

Rinse the turkey thoroughly inside and out with cold running water to remove excess surface salt.

Pat the skin completely dry with paper towels and allow the bird to air-dry in the refrigerator for 1 to 2 hours before roasting to ensure skin crispness.