Very low calorie diets (under about 1000 cal) should be supervised by a doctor to ensure they're nutritionally complete. Even then, I've never heard of it going below 800 cal.
You could cause yourself malnutrition, gallstones, hair loss, fatigue and many other side effects, including very serious ones.
You're also less likely to keep the weight off long term if you lose it through this type of crash dieting. Consistent lifestyle changes that you can keep up are more sustainable, and much safer.