Tomb of the Giants
Background
Acerinoth the Deviser was a titan wizard of vast power and unspeakable malice, the only son of Erek-Hus, King of Terror, and a death giant mother of unremarkable origin. She was part of a vast harem of slaves and sacrifices belonging to the worst of all the primordials, but through an unusual abating of cosmic hunger, the woman was spared long enough for Acerinoth's birth.
He was a wretched youth caught between worlds, as hideous as his father, despised for his half-bred weakness, but despite his mother being devoured at a young age, Acerinoth managed to escape his father's appetite by scurrying and hiding in various planes, notably the Feywild, Shadowfell and The First World. His intimate knowledge of these realms would serve him well in the coming years, when The Dawn War pitted primordial against god.
The lines were drawn and all of primordial, giant and elemental stock were called into service, even exiled Acerinoth, having developed into an imposing titan and formidable wizard. He expanded his three planar holds and lured many a doomed god, exarch and angel into their trap-haunted depths. Acerinoth enjoyed a very particular kind of execution, the prolonged and torturous kind, the kind that gave his victims a morsel of hope enough to enter yet another devious hall or chamber, on and on until their inevitable demise. With each challenge, he drained their essences with soul-crushing engines, and used this harvest to become a force akin to god and primordial in his own right. Erek-Hus quickly recognized his son's power and bestowed his mighty favor, at long last fulfilling the half-breed's existence and assuring forever his loyalty and service.
Then Erek-Hus was murdered by the combined might of Tiamat and Bahamut. The Dawn War entered its final stages, and the primordials were headed for defeat. Acerinoth, cheated of his glory, trapped in the depths of his most elaborate dungeon, under siege by Bahamut and his heavenly host, forfeited his mortal shell and sought vengeance and the eventual end of the gods through demilichdom. His dungeons were destroyed, his engines dismantled, but the evil spirit of Acerinoth endured, festering beneath the rubble of his ruined dungeon, now his tomb.
Over thousands of years, he reached out to his giant bretheren and their lesser kin, began excavating his tombs and rebuilding his soul engines. In all that time buried and conscious, his genius became touched with madness, fermented in his sadism and need for vengeance into an evil the very least his father's equal.
Adventure Summary
The followers of Acerinoth, through his guidance, have successfully rebuilt his dungeons and reactivated his soul engines, altered slightly to absorb not the essence of those gods exploring their halls, but the residual power of all the slain gods and primordials from The Dawn War over three worlds: The Feywild, The Shadowfell and The First World. All of this harvested essence is fed into a fourth tomb, built upon the blood-soaked spot where Acerinoth's father was slain, which will bring about a resurrection of both Acerinoth and Erek-Hus, a union of father and son into a new King of Terror- no, a King of Horrors- the likes of which no enduring god could withstand.
The Tomb of the Giants is really a collection of four separate titan-sized dungeons (which I think are really evocative of almost a Lovecraftian scope, eerieness and unfamiliarity), and then, if Acerinoth is thwarted in each chapter, a fifth dungeon contained within a crystalline dimension accessed via the ruby eyes of his massive skull, where Acerinoth's spirit might be destroyed forever.
Chapter One:
Steading of the Formian Duke
Set in the Feywild, the PCs are drawn into the first of Acerinoth's dungeons, the face of which is built upon the verdant surface, but the bulk of which worms the depths of the Feydark. Acerinoth's operations are overseen by a formian duke Crosnra (or whatever variastion of Nosnra you like)looking for power and standing amongst the entrenched formian kings, embracing the worship and direction of Acerinoth to further that end.
I chose formians over hill giants since they are much better suited to the Feywild/Feydark and they are much more heavily featured in 4e. A combination of scenarios from Part Three: Earth Giant Encounters and The Garden of Graves from Revenge of the Giants and Tomb of Horrors respectively, as well as the classic Steading of the Hill Giant Chief, could and should inspire encounters, though. Hill giants, ettins and ogres would be replaced with formians, cyclops, slaves and fey terrors. And DMs should really explore the possibilities of a tomb tunneled through the Feydark. It provides plenty of unique options and twists, so flavor to taste.
The ultimate encounter would be with the duke and the fey engine, and provide plenty of hints as to some 'sinister hand' influencing the formians, a map of the Shadow Rift of the Death Giant Czar, maybe even the magic chain (fits with the idea of formian slavers) that will teleport them there.
Chapter Two:
Shadow Rift of the Death Giant Czar
Set in the Shadowfell, the PCs manage their way to Moilscar, a black city of death giants in the bowels of an umbral ravine. The ruling caste count themselves members of The Bleak Court, a cult to Acerinoth consisting of necromancers, warlocks, wizards and assassins, chief among them Czar Nurgrin.
This city is so ripe for skill challenges, diplomatic and otherwise, its entirely up to the DM where he or she wants to take it. Infiltrate the court, storm the palace, dig up information on The Bleak Court and learn of Acerinoth, the enemy in the shadows, at last? It could go a lot of ways, but eventually leads to the cults headquarters beneath the palace, the second tomb. There PCs might also confront the czarina, or make a dark pact with the eager heir apparent. There are plenty of shadow and undead creatures to fill these trapped halls, as well. If at all possible, provide some sort of remorhaz-like shadow beast, something big and scary.
The final encounter consists of the Czar (a death giant or more likely a death titan), his closest retinue and the shadow engine. At this point Acerinoth should be revealed as the evil mastermind behind the giant alliance and some inkling of his plot made apparent, if not the entirety of his plans. It'll only lend urgency toward the next chapter. And if the DM thinks it wise, include the iron lever to the Hall of the Maelstrom Kings.
Chapter Three:
Hall of the Maelstrom Kings (storm, frost, fire)
Set in The First World, this is the biggest homage to the original Against the Giants, as well as Revenge of the Giants, as the enemies here are largely a collection of frost and fire giants. I decided to add storm as the third type instead of earth, just because I think we've gone beyond the ken of hill giants, ettins, etc. at this point.
A frost, fire and storm titan and their subsequent clans inhabit this third tomb, likely in themed halls, which must be traversed to reach the first engine. Borrow from Part Seven: Frost Giant Encounters and Part Eight: Fire Giant Encounters from RotG, as well as Glacial Rift of the Frost Giant Jarl and Hall of the Fire Giant King.
The final encounter in this dungeon has a lot of potential. If a DM wanted to keep in the pattern of the previous two, some amalgam of titans might be fitting, something very reminiscent of a primordial in the form of a three-headed titan of lightning, ice and fire. Or better yet, have the three titans killed separately at the end of each of their respective halls, then have Acerinoth's likeness be the three-headed primordial construct. Now the PCs face the evil mastermind (in one of three incarnations). The DM should insinuate this may very well be the demilich, and that destroying his bodily phylactery might end all this.
With the idea the phylactery is somewhere on the construct, mentioned perhaps as a disc or heart in the creature's chest, or one of the rubies of its eyes, have its defeat create a sort of China Syndrome, an overload of three elements that burns through the Hall's levels and the earth, into the Underdark. They must pursue it, a skill challenge that could take days, and potentially have PCs run into drow and other denizens in the vein of Descent into the Depths of the Earth. The remnants of the construct may become a point of interest to drow, illithids and/or kuo-toa.
If Acerinoth was not aware of who the PCs are, he is now. They've destroyed all three of his siphoning engines. In the wake of this destruction, it'd be very easy to throw in a tie-in encounter of assassins. And if you really wanted, you could have had the assassins coming since Chapter Two. Acerinoth is exceedingly well-informed and well-connected enough to pinpoint his enemies, however unwitting, and try and eliminate them. These assassins could be drow, even, further tying Underdark elements or giving proper homage to the original AtG module. The DM, early on, might even give the impression drow are behind the formian duke, a theory busted during the events in Moilscar.
Chapter Four:
Tomb of Fallen Terror
Set on a mote in the Elemental Chaos, Acerinoth has built his masterpiece tomb in the bloody mire where his father was slain. This is the dungeon I would incorporate the most of the original Tomb of Horrors, both from the original module and the 4e version, in a larger scale of course and coupled with the fourth chapter of the 4e ToH set in Pluton. High-level elementals, demons, undead, the works should be poured into protecting his most precious venture, and some very insidious traps.
The ultimate encounter would be against the giant form of Acerinoth himself protecting a Well of Destiny, Hatred's Heart, The Root of All Evil or whatever you want to call the repository all the essences were poured into. He'd be fully entact, a gargantuan lich in all the trappings. Once defeated and the repository destroyed, the module might very well seem over. It's up to the DM if he or she wants to end it for now, then have Acerinoth take his revenge weeks, months or years later. Alternatively, reminded up how Acerinoth endured despite the zeal of Bahamut and his retinue, they might continue to explore, whereupon they'd find the secret antechamber and the lair of the demilich's true, giant skull set with massive, ruby eyes. It might be that this skull leaps up and seeks to destroy the party, but I'd prefer the final chapter, myself, hinting at the magic of the rubies as a portal into Acerinoth's ruby sanctum.
Chapter Five:
Hatred's Ruby Halls
The fifth and final tomb would be cased in ruby, magical corridors compounded and multiplied by the reflection of Acerinoth's faceted eyes, sort of a delve into the demented titan's mind, with as many or as little traps and phantoms as the DM liked, but culminating in an encounter with the classic, floating skull, again of giant proportions.
Victory would be the destruction of the ancient and evil spirit and a major win for the PCs.
Expanding
I think this super-adventure would really benefit from some sort of dragon faction threaded into things. There is no love lost between the get of Tiamat and Bahamut and the son of Erek-Hus. Perhaps Acerinoth plays a dragon or dragonborn faction false in his steps towards ascension if for nothing else than to humble those of Io's blood, or maybe he cannot help but go out of his way, even at the detriment of his plans, to eradicate a draconic enterprise so filled with hate. Those might be ways to hook players in, actually, or to rehook them later on in levels. It also offers some interesting roleplaying opportunities, temporarily siding with followers of Tiamat, maybe even a notably evil dragon or two.
I'd also want to throw a nod toward Pentival, perhaps making him a dragonborn paladin or knight of Bahamut and a foe of Acerinoth in some capacity. If I were going to adopt more of Revenge of the Giants' plot, I'd have Pentival take the place of Obanar and have him the last of a line of draconic protectors set to prevent Acerinoth from every rebuilding his soul engines. It'd be doubly cool if the order once consisted of followers of both Tiamat and Bahamut, working together in the spirit of Io and for their mutual survival. Of course that organization wouldn't last forever, hence Pentival being the last member and Acerinoth able to finally reconstruct his dungeons/engines.
After the PCs emerge successful from the Steading of the Formian Duke, Pentival would contact them and begin an ongoing quest line much like runs in RotG, enlisting them to take down these other installations.
And there you have it, my fusion of Against the Giants/Revenge of the Giants and the incarnations of The Tomb of Horrors.