Katashka thrives on mortal fears of death and the undead. He is thought to have brought the first undead into the world, and certainly created the first liches and dracoliches. Katashka’s servants are part of the Lords of Dust, and his prakhutu is the dracolich Mazyralyx (thought by some to be the origin of many myths of the Keeper).
The location of Katashka’s prison is a mystery—the
Lair of the Keeper in the Demon Wastes is one possibility, but it’s just as likely that the lair instead contains
a connection to his heart demiplane. Katashka’s cults
are more widespread than many other overlords, which
suggests his prison could have been shattered and
scattered like that of Rak Tulkhesh. As Katashka is
known to create liches, one possibility is that pieces of
his shattered prison are used as phylacteries by his lich
champions, who spread his influence wherever they
go. Katashka largely works with undead as opposed to
fiends; his champions include the ancient wizard Kyuss
and his spawn.
Source: Chronicles of Eberron
Lord of death and undeath, Katashka is the master of the most ancient undead creatures. In addition to rakshasas, he is served by vampires, immoliths, and a number of dracoliches—corrupted champions of the first great war. His chief agent is the runescribed dracolich Mazyralyx, said to be the first dragon to fall to this fate and possibly the inspiration for some myths of the god known as the Keeper, one of the Dark Six.
Source: Eberron Campaign Guide
The most terrifying powers in Eberron are the overlords of the Age of Demons: the first children of Khyber. Each of these spirits rivals the deities of other settings in sheer power -- and even while imprisoned, they can touch the world through dreams and the actions of their servants.
The overlord Katashka possesses power over death. In the ancient battle against the dragons, Katashka raised a host of horrors, and those that survived the war are now the oldest undead creatures in existence. Katashka was imprisoned at the end of that conflict, but a number of his rakshasa servants still remain at large among the Lords of Dust. At the DM's discretion, these malefic spirits may have any of the new sorcerer/wizard spells presented in Libris Mortis, or possibly even levels in the pale master prestige class. A wizard can learn this magic by dealing with the rakshasas, but what price will he have to pay?
Katashka is not truly a god, so he cannot grant spells. However, he can guide the thoughts of those who worship him and help them to draw dark magic from the heart of Khyber itself. Death cults have appeared throughout history -- the Dhakaani records speak of a sect of necromancers wiped out by a powerful emperor, and several stories of human death cults predate the rise of Galifar.
Katashka's cultists gain access to the Deathbound, Undeath, and Evil domains. In addition, people born in the vicinity of his prison may be further touched by his darkness. It's up to you as DM to decide exactly where he is buried, but this concept forms a good basis for the Tomb-Tainted Soul feats.
Any of the gods presented in Libris Mortis could be adapted as overlords of the first age, though that status represents a step down for most of them. Doresain would be an especially appropriate choice -- a party exploring a ruin from the Age of Demons could be surprised by zakyas with the powers of gravetouched ghouls!
Source: Libris Mortis Expanded, Part One
You cannot defeat me. You fear death, knowing your flesh will rot and your soul will shrivel away in Dolurrh. But I serve the Grim King, the Keeper of the Gate— and even if you kill me now, he will open the gate for my return.
The overlord Katashka embodies the fears of both death and the undead. People often assume that the followers of the Blood of Vol want to become undead, but it is Katashka’s cultists who embrace this dark destiny. His loyalists dream of a world where the dead reign over the living, reveling in the power death holds over the rabble, secure in the belief that Katashka will either grant them immortality through undeath or raise them when they fall.
Katashka thrives both on the fear of death and fear of the undead. As such, his cults might spread plagues or otherwise cause mass death, or they might unleash ghouls in the sewers or trigger zombie outbreaks. His intent is not death itself— the Grim King draws strength from the fear of the living. As such, Katashka rarely seeks to kill people swiftly. The primary purpose of the lingering plague is to sow fear among those fighting it, and the small pack of ghouls that causes long-term terror is more effective than an army that wipes out a city in a day. Likewise, Katashka’s undead champions revel in their unnatural condition; they want people to know what they are and to fear them.
The Gatekeeper’s cults are often transactional, with cultists serving the Grim King in exchange for secrets of necromancy or undead servants. Corrupted cults will in some way tie to death or the undead; Katashka also produces revenants. Katashka’s cults have no established territories, but are actually rarer in Karrnath than elsewhere in the Five Nations, as the Blood of Vol presents a different path to necromantic magic along with a more positive overall message for followers.
There are some obvious similarities between Katashka and the Keeper, and some scholars assert that they are one and the same—that Katashka’s actions in the Age of Demons inspired the myths of the Keeper. A critical difference is Katashka’s focus on undeath, in some ways resembling the role of Orcus in other settings. In contrast, priests of the Keeper also deal with general greed, offering deals that have nothing to do with death or undeath.
Forces. Katashka’s forces are undead; even within the Lords of Dust, his champions are liches, death knights, and dracoliches. His transactional cults include necromancers, though he actually has fewer necromancers in his service than are found among the Blood of Vol and the Emerald Claw; his followers don’t need to understand the science of necromancy if their dark lord raises bones for them.
Gifts. Katashka’s treasures are tools that consume life, such as a sword of life stealing. His cultists may also possess tools that create or command undead. Beyond this, the greatest gift Katashka grants his followers is life after death. A champion of Katashka could be repeatedly raised after death, forcing adventurers to find more elaborate ways to prevent their resurrection. His champions could also be returned as undead; only valuable servants are granted a second life as liches or vampires, but lesser cultists could rise after death as zombies or ghouls—an unpleasant surprise for adventurers fighting them!
Character Ideas. A Zealot barbarian dealing necrotic damage could have a tie to the Grim King; do they have to earn this power by killing in the name of the Gatekeeper? The Undying warlock patron is another sound match for Katashka. While the Death domain and the Oathbreaker paladin both fit Katashka thematically, Katashka isn’t a god and such characters may be a better fit for the Keeper or the Blood of Vol. Perhaps you died in the past, then were returned to life by Katashka, but with a bargain to fulfill—along with the knowledge that if you fail to carry out your mission, you will die again. Will you do the bidding of the Grim King, or will you try to find another way to earn your life?
Story Ideas. Any plotline involving evil undead or powerhungry necromancers can be laid at Katashka’s feet; his cults require less nuance than the Blood of Vol. A common theme is the fact that Katashka doesn’t simply want to kill innocents, he wants people to fear death and the undead.
Source: Exploring Eberron