If you chose to 'open it', embracing the 'evil' ending, Professor Fig will be killed by the Ranrok Dragon in the cut-scene before the fight. This is a blink-and-you'll-miss-it moment, with a massive pile of debris being walloped onto your kindly mentor. After the fight, the player-character will absorb the dark energy and their eyes will glow red for a brief moment. This seems to imply that your character has become more powerful and could potentially go down a dark path in future.