Complex sentence: The only logical conclusion is that Elizabeth and Dr. Leon diabolically placed the mummified corpse of Isabella Medina in the crypt, who had been entombed alive by Sebastian Medina decades before and suffered final agonies of insanity and starvation causing the horrific appearance of her corpse.
Split into simpler sentences: The only logical conclusion is that Elizabeth and Dr. Leon diabolically placed the mummified corpse of Isabella Medina in the crypt. Indeed, she had been entombed alive by Sebastian Medina decades before and apparently suffered final agonies of insanity and starvation causing the horrific appearance of her corpse.