Gellert was rather proud of his self transfiguration – his inched up the cliff face in starfish form, his tentacles suctioning to the rock with a grip that defied the waves which smashed against him with earth shattering booms. It took a long time, but he reached the top and found a ledge that would sufficiently hide him. He transfigured himself back to human, then into a rat and scurried across the island. The rat was faster, but its fur was no protection against the sea spray and the frigid cold of the residual dark magic had his body soon crusted with ice.
He scurried around simple grave stones, avoiding the path as he crossed the space between the sea and the prison. The large portcullis was closed but it posed no obstacle to him in this form, or truly in any other. The trick to this mission though would be to not leave the prison any more vulnerable than it already was.
The warm glow of the patronus indicated the presence of a guard, one among the many that now guarded this place. He paused, shrinking into the shadow and returning to his human form. The patronus was a rooster, which clucked around, pecking at the floor and walls. Trailing behind it was the auror – a man of roughly Gellert's height with black hair and beard, perhaps of Slavic descent. He would do nicely.
A wordless charm later and the guard crumpled, his patronus fading into nothing. Its absence did not affect Gellert, his own innate darkness in proportion to that of the prison. He levitated the unconscious man into the shadows where he hid, then tore out several hairs to add to his polyjuice. He drank the sienna liquid, grimacing as his features shifted, then transfigured his clothes to match the other man.
Minutes later he was striding out down the corridor as though he was a guard.
The prison was incredibly understaffed; perhaps in peacetime the auror corps had reduced their numbers because back in his day there would have been more than enough to cover this prison and fight the necessary battles. It was an entire dose of polyjuice before he came across another guard, conjuring a ghostly white rooster as they closed on one another. His rooster did nothing to alleviate the residual cold of the dementors but the other guard wouldn't notice that. They nodded as they passed one another but otherwise continued in silence.
Gellert passed into the high security area, a guard at the checkpoint waving him through with a passing glance. The performance really was quite poor.
He found Sirius Black in one of the topmost cells; the man was a shivering wreck, huddled beneath the robes he had worn on the day he was arrested. He was surrounded by other death eaters, all of whom were as affected by the darkness as Gellert and chatted merrily among themselves. As he strode down the corridor, the conversation turned to leers and the inmates shouted abuse at him, promising that their dark lord would come for them, that he would kill them all. He levelled his wand at one of them; Dolohov had tried to curse Hermione, but first, Lucius had informed him that the man was a brilliant spell crafter.
Grindelwald plunged into the man's mind, shredding his defences like paper and tearing away the secrets of his dark creations. Yes, that purple spell was rather nice, as was that unfinished white one. Gellert knew exactly how to perfect that one.
When he withdrew, the death eater was little more than a whimpering wreck on the floor. The inmates had gone silent. A flare of violet light later and Dolohov's blood was soaking the stones.
'Death eaters...' He called out to those in the corridor. He waved his wand, removing his disguise so that they could see exactly who he was. 'You know me as Gellert Grindelwald; a pure blood, head of the ancient house Grindelwald which traces its lineage back to long before Salazar Slytherin.' He had their attention now, and the familiar words of his doctrine rolled off his tongue, speaking of educating muggle borns and preserving magical blood. He did not expect followers to join him from among this rabble; most of them were too stupid to see reason or mad from decades under the dementors.
Unsurprising nobody pledged allegiance, so he blasted the door of Sirius' cell and handed the shivering man the wand he'd stolen from the guard downstairs.
'Grindelwald?' The man asked curiously and he nodded. Energy seemed to surge through the wizard and he slipped into dog form, the four legs seeming to strengthen him. They were just in time, a shout echoed up the dripping corridors and somewhere an alarm started ringing. Somebody must have come across the unconscious auror.
They jogged down the stairs, Sirius' claws scratching against the slick stone. Gates were magically slamming shut, but such physical barriers were hardly an obstacle. Gellert slashed his wand through the air as they approached each one, the locks blowing off. They encountered the first guard at the bottom of the stairwell and Sirius pounced, the weight of the large black dog throwing the auror off balance. The two tumbled to the floor in a mess of snapping teeth and claws and the auror's head hit the stone with a sickening crack. Gellert dragged Sirius out of the tangle by his scruff as he jogged past and they darted down the next flight of stairs.
A purple curse from Gellert's wand had the auror at the checkpoint falling against his companion, who struggled to catch his petrified teammate before he smashed on the floor. Half a second later both were falling, a last minute cushioning charm preventing any further damage as they whipped around the corner. A blasting hex later and they were dashing through the bottom levels of the prison, Sirius took down another auror and they were out in the fresh air. Gellert skidded to a halt half way between the land and the sea, a stunning spell taking out a pursuing guard.
'Cover me!' He bellowed to Sirius, who had also skidded to a halt. The wizard transformed back into a man, his stolen wand already in his hand.
Gellert raised his wand in one arm, an athame in the other. His magic surged outwards in a wave, burrowing down, searching. He found them easily enough; those ancient and terrible wards, cast by Ekrizdis centuries ago. He knew them intimately, having used them as inspiration for the wards upon his old prison. He sliced his palm, blood spraying across the ground as he called for the wards to rise, to imprison, to answer to him.
Sirius Black faltered in his casting, matched by the aurors he duelled. Everyone felt the stirring of the dark, terrible magic beneath them, like an ancient beast waking from a slumber. Gellert felt the wards brush over him, probing his power and testing his control. For a moment there was silence, neither side casting a single spell, waiting as the wards flexed against their new master. They must have found him sufficient, because suddenly they were roaring upwards in a surge of fire, curling over the prison and colliding to complete the dome with a crack of thunder.
Aurors cried out, stumbling back from the wall. One man seemed to recognise the danger of being trapped within and lunged forwards as if to escape. He almost made it, then a tendril licked his leg. He aged before their eyes, mid leap, skin wrinkling, hair greying, muscles atrophying and sucking into his skeleton. Then before their eyes he disintegrated into dust.
Gellert didn't wait for the flames to fade, grabbing Sirius' arm with his still bloodied hand and plunging into the sea. He managed to transfigure himself into a fish as they fell; hitting the water on his flat side with a force that felt like it had snapped several ribs. Sirius yelped beside him, in dog form, although how he thought that would help baffled Gellert. Spells zinged around them, hitting the water with bursts of steam. Black seemed to get the idea, quickly transforming back to human and managing to turn into half a shark. He had the head and front fins though, which were the important parts, so the two wizards darted off, fish forms sending them surging through the dark turbulent water and away from the prison.