In a small bowl, combine minced garlic, ground cumin, ground coriander, smoked paprika, ground turmeric, ground cinnamon, ground allspice, cayenne pepper, kosher salt, and black pepper.
In a large bowl, whisk together Greek yogurt, fresh lemon juice, olive oil, and water until smooth.
Add the spice mixture to the yogurt mixture and stir until well combined.
Add the chicken thighs to the bowl and toss to coat evenly with the marinade.
Transfer the marinated chicken thighs to the slow cooker, spreading them out evenly.
Cover and cook on low for 6 hours, or until the chicken is tender and cooked through.
Once cooked, remove the chicken from the slow cooker and shred it using two forks.
Return the shredded chicken to the slow cooker and stir to mix with the juices.
Serve the chicken shawarma warm in pita bread, over rice, or with salad and your favorite toppings.