Step 1

Bring lemongrass, garlic, ginger, oil, and achiote seeds to a simmer in a small saucepan over medium heat. Reduce heat and simmer gently, stirring very occasionally, until oil is brick-red and fragrant, 30–45 minutes.

Step 2

Do Ahead: Achiote oil can be made 1 week ahead. Let cool; cover and chill.

Step 3

Combine lemongrass, garlic, shallot, ginger, vinegar, Sprite, and 1 Tbsp. salt in a large resealable plastic bag. Whisk to dissolve salt. Set chicken legs, skin side down, on a cutting board. Working one at a time, cut a ½"-deep slash along both sides of thigh bone with a paring knife (this lets the marinade penetrate and helps the meat cook more quickly). Add chicken to bag and close, pressing out as much air as possible; turn to coat. Marinate at least 6 hours and up to 12 hours.

Step 4

Prepare a grill for medium-high indirect heat (for a charcoal grill, bank coals on one side; for a gas grill, leave one or two burners off). Remove chicken from marinade and secure each piece with 2 skewers if desired (the skewers will help you maneuver chicken on the grill without tearing the skin). Season lightly with salt. Discard marinade.

Step 5

Grill chicken over hottest side of grill, turning occasionally, until deeply browned and charred in spots, about 5 minutes. Move to cooler side of grill and cover. Continue grilling (covered), basting with achiote oil and turning every 5 minutes or so, until skin is burnished and meat is cooked through, 30–40 minutes longer.