Instructions

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.

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

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.

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.