Complex sentence: If a Mohra is killed, it will return to life, much stronger and bigger than before, and the only way to destroy them for good is to "bring darkness to (its) thousand eyes"-destroy the jewel on the forehead.
Split into simpler sentences: If a Mohra is killed, it will regenerate itself, returning to life much stronger and bigger than before. The only way to destroy a Mohra permanently is to "bring darkness to (its) thousand eyes"--that is, destroy the jewel on the forehead.