Place the boneless chicken breasts in the bottom of the slow cooker.
Pour the enchilada sauce over the chicken.
Turn the chicken breasts several times to coat them evenly with the sauce.
Cover the slow cooker with the lid.
Cook on high for 4 hours, or until the chicken is fully cooked and tender.
Once cooked, shred the chicken directly in the slow cooker using two forks.
Cut the yellow corn tortillas into bite-size pieces.
Add the tortilla pieces to the slow cooker.
Stir gently until the tortillas are evenly combined with the shredded chicken and enchilada sauce.
Add 1/2 c of the shredded cheddar cheese.
Add half of the drained black olives.
Stir until evenly distributed.
Use a spatula to spread the mixture into an even layer.
Sprinkle the remaining shredded cheddar cheese over the top.
Add the remaining black olives.
Cover the slow cooker again.
Cook on low for 45 minutes to 1 hour, or until the casserole is hot throughout and the cheese is melted.
Serve warm.