The darker eye is Graves’. Graves was the one with the visions, not Grindelwald, and he proved… difficult about them. Disinclined to share. But that’s fine; a prophet’s power is in their voice but it’s the eyes that are the key to a seer’s power, and if you know what you’re doing it’s easy enough to steal it. It takes Grindelwald a while to set the ritual up and when Graves realises what he’s doing he fights Grindelwald with everything he has.
Of course he does. You think Graves would just let a dark lord have that sort of advantage over the world? Grindelwald’s too powerful and too cunning for Graves to fight openly, but a seer’s power is in the eyes and Graves can sabotage that. Even with magic-inhibitors on, even drugged and clinging desperately to reality to keep it from slipping away, Graves has enough control to channel his magic to his eyes and - well. It’s surprisingly easy to set your anger on fire if you have enough of it. Painful, but easy enough. Visions flicker through the flames and Graves sees the world spinning away from him, sees Grindelwald striding through his life wearing his face, sees Tina sentenced to death and a hunted boy dissolve into black-red-black and his city torn to shreds. He sees might have beens and might still bes and he sees himself burn to death in one, sees Grindelwald tire of him in another, sees himself scrawl out a bomb in runes of his own blood and bare his teeth in a bloody grin when Grindelwald steps on the trap and Graves brings the roof down on both of them -
In one potential future, Graves sees a man with curly hair and a hesitant, nervous smile. The man holds out a hand and the Graves in that vision takes it and pulls himself to aching feet. It’s the only vision where the sun shines.
It’s the last thing he sees.
Grindelwald roars in anger, forcing Graves’ head underwater until he chokes and suffocates. Graves wills the fire to keep burning but the water drowns it and lack of air is making him dizzy and weak. Ice floods his lungs and sends shooting pain through his chest. The fire goes out.
The eye that Grindelwald salvaged is damaged, the vision blurred and marred by dark shadows that the other eye can’t see. It shows him hazy visions of a future wreathed in flames; it aches and burns and digs like knives into his brain. He should have known better than to steal a seer’s power, should have known that nothing of Graves’ would serve him willingly, but Grindelwald has always been a man that cares more for the prize than the cost to win it. He takes his mismatched eyes and the grudging glimpses of one day soon. He sees futures that dissolve into anger, into war; he sees himself victorious and bloody, surrounded by chaos, and he marches on with false security to a vision that will never come true.
He should have known better, but he thinks Graves is a broken man. Graves’ visions will lead him merrily to his death and Grindelwald will run towards them with eager triumph in his strides.
In his cell, Graves bares his teeth in his bloody grin and holds tight to the only future he saw where the sun will shine again.