The names of the zombie horde made it pretty clear what he was dealing with:
DETERIORATING INFERIUS
Undead, Level 3
Well, Harry had been correct that the guardians would be about the right level for him. He wasn't sure what level a new inferius should be, but these had clearly diminished down to the level of his third-year main quest after centuries posted up in Azkaban. Unfortunately, that didn't account for the sheer number of the creatures. The swarm of red dots popping in from the edges of his map was hard to count in the zombie apocalypse moment of horror, but it was probably dozens. Maybe a hundred.
And he'd never know whether that number would have been a lot fewer if he'd come here without a bunch of much-higher-level helpers. It couldn't, right? It just wouldn't make sense for the world to retroactively change the number of zombies that Ekrizdis had created five centuries earlier to provide an appropriate challenge for an archmage, a defense teacher, an auror, and the Boy-Who-Lived. Right?
The inferi were slightly-mummified, their clothing of an ancient make but still holding up like all the other furnishings in the nearby levels. They were almost entirely adult men of nearby European stock, wearing garb that seemed vaguely nautical (for all that styles had changed since the 1400s). It was an easy bet that these were the remains of the sailors that had disappeared into Azkaban over the years.
"Bubble head charms!" Remus realized, as everyone began to ready their fire-making incantations. Fortunately, he'd considered the possibility that a lot of fire thrown around in an enclosed space could have ramifications for the amount of oxygen available, and taught Harry the spell in passing.
Each of the men wrapped their heads in a somewhat-silly-looking globe of air, and then began to cast at the shambling horde. Harry was just glad that he'd gotten dispensation to cast spells in Azkaban (for all that the Trace probably wouldn't notice him there for a whole list of reasons, even without instancing).
Harry's screams of "Incendio" managed to launch accurate gouts of flame to ignite nearby undead. Remus and Dawlish were also peppering in a few shouts of "Confringo" between their own fire-making charms, and the blasting curse was highly-effective at exploding zombies in flaming bursts. Harry hadn't quite gotten his head wrapped around that spell, but it seemed worth learning. Both defense professors' magical stamina bars dipped visibly each time they cast it, so it was clearly not an all-the-time spell.
Dumbledore had backed up to let them handle the first wave as he worked up to something ridiculously powerful. Harry could see his magical stamina dipping as he wove a complex conjuring. A few moments later his wand lashed out, crimson and gold flames spilling out of it and circling them in a ring of intense fire, the heat directed outward. "Move forward," the headmaster suggested, simply, as the inferi started to balk and retreat in the face of so much fire magic.
Harry was really glad of the charm allowing him to breathe clean air, as the immense room very quickly filled with the smoke of charred zombies as they strode through the space, igniting every creature they could see. Each of the inferi shattered into flaming debris and dust as their hit point bars emptied, the smoke of their passage incredibly foul-looking.
Within a few minutes, they were basically just hunting down stragglers, and Dumbledore didn't even bother to maintain the defensive wall of fire, instead launching his own gouts of flame: his fire-making charms were like a welding torch, a bright white lance of flame slicing deep into the undead. While everyone's magical stamina was low after the last inferius fell, none had gotten close to laying a withered hand on them.
766 Combat XP Earned
OWL Combat Magic XP Earned
OWL Charms XP Earned
That was a lot of XP. But, given that just beating a level 3 student in a duel was worth 45, it seemed like mowing through a hundred inferi should have been worth over a thousand? He must be getting some kind of penalty for being the lowest level member of the party, particularly with more than a level difference like he usually had with the other Marauders. Though it was probably fair; he definitely hadn't done a quarter of the work.
The headmaster waved his wand a few times, using wind charms and some kind of advanced cleansing spells to disperse the smoke and restore oxygen to the area. In a minute, all that was left of the fight was the piles of too-black ash vaguely in the shape of bodies. In places where they'd destroyed several together, it wasn't even recognizable as more than a carpet of soot.
Remus tentatively dismissed his bubble head charm and took a guarded breath, then nodded to the others. "Doesn't smell great, but it should be safe to breathe."
"Such a tragedy," Dumbledore said, regarding the destroyed undead. "Kidnapped, experimented on, and then made into such mockeries, all for nothing."
Harry blinked, just suspicious enough of the old man after the last year to wonder if he'd have been less disapproving if Ekrizdis had more success. "They must have been guarding something," was all he said, however. "Maybe a basement laboratory."
THE PRISONER OF AZKABAN (MAIN QUEST)
A secret from your past has escaped.
* Get to Hogwarts
* Learn about the crimes of Sirius Black
* Learn about Dementors
* Learn more about Ekrizdis
* Investigate Azkaban for what Ekrizdis left behind
* Defeat the guardians
O Descend to the basement laboratory
O Learn the goals of Ekrizdis
The stairs down were hard to miss, once they walked to the far side of the floor. A spiral staircase stood near the courtyard wall, wide enough that all of them could walk side-by-side. It was even shallow enough that it would have been ideal for carrying down equipment, perhaps. As Harry glanced down at it, it spiraled down on his map, seeming to go on all the way down to the base of the tower.
"I know now that we should've been more curious," Dawlish observed, "but there's no way we'd've missed a hidden staircase this large punching through the rest of the building."
"Space expansion," Remus figured. "Might be narrow enough from outside to just seem like a ventilation shaft or load-bearing column, between rooms."
Harry nodded as they began to descend through unspoken agreement. The map of the lower levels was doing some weird stretching the way it had other times he'd been inside a building or room that was bigger than it was supposed to be. If he completed the map of the lower levels, he was sure it would, indeed, have a barely-noticeable shaft were the large stairway should be.
"I guess this is one way to keep us from figuring out how to get to the basement," Dawlish grumbled after they'd already descended a dozen floors.
"Clearly Ekrizdis did not wish for visitors," Dumbledore agreed. "But with the connection directly to the hidden living area of his followers, likely there was a way past the inferi. Perhaps it needed his presence, or some kind… of…"
Whatever speculation the old man was about to make about how the dark wizard's apprentices might have bypassed the zombies was lost as their lights played across the first of the bodies.
Little more than a mummy, the figure was still identifiable as a light-haired man in linen robes. Just as none of their furniture had been subject to bugs and microorganisms, five centuries had seemingly just been enough for the apprentice to desiccate. His head was facing them, his body face up, as if he'd been descending the stairs and falling backwards.
Or been running up the stairs only to turn to face an attacker.
"Killing curse?" Remus wondered. "I don't see a mark on him."
Dawlish shook his head and explained, "I've seen that pose before. Dementor's kiss. There a… there's a way they curl up to try to protect themselves before they go limp." If anyone in the auror department wasn't disturbed by the worst method of capital punishment in Britain, they would tend to find themselves quietly moved to another department. The Dementor's Kiss was legal for the worst crimes. That didn't mean it was good.
Shedding his normally-affable mien, Dumbledore looked every one of his 111 years. His brain had jumped to the idea that, "If Ekrizdis accidentally created dementors, and none of his people were prepared… this will not be the first body we find. Harry, I…"
"I'm okay, sir," Harry allowed the old man to abandon his attempt to talk him into staying behind. After all, Dumbledore knew they probably needed him to see the quest through, regardless of the effect it would have on his childish sensibilities. Hopefully, Harry's mental fortress would prove up to the task of preventing the nightmares.
"We'll… uh… we'll get them on the way out?" Dawlish decided. "Probably a lot of families who'd like to finally put a body in the tombs of their family members what went missing all those years ago."
And they, indeed, found more as they descended, each in a similar pose. Perhaps a dozen had made it to the stairs. Given the number of bedrooms upstairs, they expected to find even more beneath. As Harry's map suggested they were now about a dozen yards beneath the entry level, the stairs finally opened up into a room.
It was smaller than the rest of the width of the tower, mostly taking up the space beneath the courtyard. Uneven fog at the periphery suggested to Harry that there might be hidden rooms filling the rest of the footprint around them, but they'd exited onto a room nearly the size of a quidditch pitch. A cleverly-vaulted ceiling reached up to keep the courtyard from crashing down into the room without the use of columns. And, within, an immense ritual circle was carved into the stone of the floor. There were so many rings, none of them a precise circle but instead tracing unusual parabolas that itched at Harry's mostly-untapped skill at arithmancy. Along each, countless runes, gems, and precious metals had been embedded into the floor.
As the headmaster's conjured lamps spread out to light the space, they tried not to pay too much attention to all the dementor-kissed mummies cowering around the edges of the room.
"Seven significant rings," Dumbledore observed almost instantly, discounting the binding and balancing circles within the diagram. "If I'm not mistaken, each has seven focal points…"
"Forty-nine apprentices," Remus realized. "Not fifty. Seven sets of seven. Just like the number of floors. He began this ritual as soon as he had enough apprentices to do it."
"And the master, himself, in the center," the headmaster nodded. "Perhaps they used this for many great rituals… but my intuition is that they used it once and it went badly wrong."
Dawlish, not nearly as academic as the two other adults in the room, was simply muttering to himself as he tried to count the bodies. "No statute of limitations on murder…"
"I don't think this is the laboratory," Harry noted, realizing his quest hadn't checked off. He'd stayed well clear of the outer ring of the oversized ritual circle and the bodies, but had managed to map most of the room as he walked around the circumference. "There are some stairs down over here… and I think there might be a tunnel out over there." Unsurprisingly, the two other exits from the room were at the center of the other sides of the tower's interior triangle.
"What direction's the tunnel out?" Dawlish asked, interested.
Harry pointed. "Basically northeast, I think. Far side of the tower from home."
The auror nodded. "We've seen dementors coming and going from near the water down that side, but nobody could ever find a tunnel. Must be hidden by a powerful illusion on the outside."
"What were they even trying to do here?" Remus asked, fascinated by the complex circle. "This is beyond my understanding of arithmancy, but I'm seeing a lot of rune clusters common in dark arts."
"I would require much longer to examine it to be truly sure," Dumbledore admitted, "but if I may make an educated guess… I'd suspect this was about passing some unique capability Ekrizdis possessed to his apprentices. And it may have had unintended consequences."
Remus and Harry nodded. The headmaster had been read in on the idea that Ekrizdis had been trying to give the game system to his apprentices. It made sense. Dawlish barely seemed to notice the conspiratorial agreement among the other three. "I think I had a several-greats aunt go missing in the 15th century. Wonder if she's here, somewhere."
"More bodies in the tunnel," Harry frowned, having drifted over to the presumed exit. "It's like they were trying to get out any way they could."
Dumbledore asked a seeming non-sequitur, his voice audible even in the giant room due to the lack of other noise and excellent acoustics, "How many dementors would you say there are in Britain, John?"
"Well it's hard to get an exact count, since they look so similar and they come and go from the insides of the tower as they please," the auror mused. "But if I recall correctly based on the number that were tasked to me for Black, and the number that needed to be held in reserve at the tower, the general theory is that there can't really be much more than a few dozen." Before he could ask why the old man was asking, he finally got it. "Yeah. There might well be around forty-nine of the blighters. Are you thinking…?"
"That Ekrizdis accidentally caused his apprentices to become—or, at least, release—dementors upon their demise? Yes," Dumbledore said. "Unless something else bears investigation? Then I suppose there are now only the stairs down…"
Reconvening at the stairway, the four shared a glance and resumed their normal formation, the defense instructors in the front to protect the headmaster and the Boy-Who-Lived as they walked down the less-elaborate staircase. It was a simple structure like the stairs elsewhere in the tower: just wide enough to move larger magical equipment down a floor.
As Dumbledore's floating magical lights spread across the second basement, there were cries from each of them of, "Expecto Patronum!" For, sure enough, the floor seemed a little more broken up into various laboratories, but there were dementors clearly visible from the exit of the stairwell. The three creatures—black robes to everyone but Harry, who saw the shimmer of broken user interface—seemed to have posted up around a segment of the floor that was lit by its own blue glow.
A subsection of the level was warded, a perpetual barrier that kept the dementors from approaching the figure within. Perhaps they had been waiting there for five centuries, hoping that the ward would fail and allow them access to their creator. Because, even though his user interface was flickering slightly in the presence of the dementors, Harry could still see the floating name and details of the figure protected and trapped within the wards.
EKRIZDIS OF LAMIA
Lich, Level 20+
[CHILDREN OF THE ARK (MASTER)]