In my Dune-based campaign, the residuum is effectively the spice. Here're the rules regarding usage. Skip down to "Using Residuum as a drug" if you want to bypass the flavor text.
Using Residuum
[Note: These rules are intended for mature players seeking to simulate a very particular facet of Herbert's literature- the fact that the spice was a highly addictive geriatric drug and a poison. It behooves the responsible party of any gaming group to recognize the implications of simulating drug use and withdrawal in the presence of minors and is not encouraged by the author of these rules. These are optional rules which are not required to run the setting accurately.]
Traditional Methods of Ritualists
When a priest burns sanctified incense to perform a ritual, she enters a meditative state in which she can petition the powers of the Astral Sea for supernatural assistance. Transcending the mortal sphere, the priest becomes an agent of the supernatural decrees which state, quite clearly, that the power of the divine shall be made manifest in the realm of mortals.
When a wizard uses rare components, typically aligning them in a manner that matches the complex patterns in their spellbooks, the pattern creates a cipher which, imprinted upon a trained mind, presents clarity in the complex cryptography of the arcane iconography that weaves in full across the many pages of a spellbook. With this cipher, the codes within the images unravel, and the wizard can manipulate the strands of magic per the ritual's complex instructions.
Residuum as Substitute
Ingesting residuum in minute doses allows ritualists to perform these same tasks. Priests transcend the conscious world to ask for supernatural assistance without the need for incense and incantation, while arcanists find the patterns within the pages of their spellbooks instinctively as their minds are reshaped for the task.
Typically, divine ritualists ingest residuum in the form of small, formed pieces of unbaked dough or light wafers. Arcane ritualists often carry very small vials of perfectly measured residuum mixtures; different orders swear by different mediums.
Residuum as a drug
A considerable portion of the nobility, not being ones to be left out of a major cultural movement, took to residuum usage as a regular part of their regimen. The general consensus is that small, regular doses of residuum slow or stop aging and also provide moments of clarity. This consensus is supported by observation of those who take residuum regularly--members of the nobility are usually in better health than the peasantry, and those nobles who ingest small rations of residuum with their food retain their youthful vigor for decades longer than they would normally. These effects are clearest amongst humans and halflings, whose typically brief lives (relatively speaking) are given precious years with dedicated usage.
In her memoir View from the Sun Throne, Empress Hesliate Brenstroke referenced these effects saying:
"They [the nobility] wear their young faces to court as though they were attending an eternal masquerade. The greatest minds of the last generation look bright-eyed through the visage of their young adulthood, and some who began a regimen in their childhood sit upon the benches in the Eternal Garden of Pelor's Light watching the next generation at play."
Of course, were an explorer to return from the wild with news of a Fountain of Youth, there would be some small group decrying the find as diabolical. So too are there concerns amongst some that residuum is an insidous drug, whose addictive properties are often masked by the reasonable desire of the user to enjoy the very palpable benefits given. There is evidence that residuum does have a very potent addictive quality that can cause dangerous bouts of withdrawal with those who cease usage, and students of these effects (thorough tests have been performed at the Murus Sorcerous) suggest that this withdrawal is the body reverting to its natural state, aging as time catches up with the user, and the mental pains associated with the withdrawal is the reordering of the mind to a less magicked, less ordered state.
Inhalation of residuum in its powder form is usually lethal, which philosophers ascribe to the damage it causes to one's natural vapors, filling the lungs with blood and mucus as an unfortunate side effect.
Mechanics / Rules for Ingesting residuum as a drug
0. These rules introduce three new conditions: Residuum Use, Addicted, and In Withdrawal. These conditions follow the rules below, and cannot be bestowed, removed, or manipulated by any power or ritual that does not specifically reference these conditions by name.
1. Consuming 20 gp of residuum within the span of 7 days grants the Residuum Use condition for 7 days, and triggers "3. Rules for Addiction" (possibly granting the Addicted condition)
Characters with the Ritualist Feat are practically guaranteed to gain Addicted Status given regular ritual use unless they expend time hunting down traditional components.
2. Residuum Use Condition:
+1 bonus to passive insight checks and perception checks. This increases to +2 at the paragon tier, and +3 at the epic tier.
+1 bonus to initiative.
Residuum users cease aging for up to 50 years, dependent upon their a strict adherence to the regimen.
3. Rules for Addiction
Make an immediate save against poison. Failure indicates addiction.
4. Addicted Condition
If the character has ingested 20 gp worth of residuum within the last week, there is no negative effect.
If the character has not, the character's condition moves to In Withdrawal.
5. In Withdrawal Condition
If the character In Withdrawal takes 40 gp of residuum within 7 days, he moves back to Addicted status. This increases to 400gp in the paragon tier, and 4000gp in the epic tier.
Once per day, a character who is In Withdrawal must save or lose a healing surge. Healing surges lost in this fashion do not return so long as the character is In Withdrawal.
These saves happen during extended rests, or if not that, dusk. If combat is to ensue during these times, roll the save prior to initiative.
Also, please note that these are not saves against poison.
Heroic tier characters who pass five saves lose their In Withdrawal status. Paragon tier characters must pass 6 such saves, and epic tier characters must pass 7 saves.
If a character In Withdrawal fails an In Withdrawal save and has no remaining healing surges, he immediately falls to 0 hp and the dying rules come into effect.
6. Dying from Withdrawal
Characters who die from their withdrawal, lose all of the status effects above if their are raised from the dead through magical means.
People who die from withdrawal without receiving the appropriate blessings and burial rites of the Raven Queen suffer a similar fate as those who do not receive such blessings- they rise in undeath.
Acknowledgments
The First Acknowledgment
It is every player's choice to either embrace residuum as a part of the setting and a part of their characters, or reject it - due to play style, personal tastes or reasons, or simply weighing the benefit versus the risk of the mechanics.
The Second Acknowledgment
It is certainly the hope of the campaign design that players will elect to embrace residuum usage as one of the challenges, and drives of the campaign- given how central the stuff is. [Note:This is something of a given- no one writes this amount of information down with a hope that everyone says "thanks but no thanks" to it.] Being wary of using the stuff is similar to being wary of commiting violent acts in the setting of Conan, to use an analogy.
The Third Acknowledgment
The implications of these rules are understood by their designer. Here is a list of them:
1. Dwarves are very resilient to addiction, but this translates in a greater ability to dabble in rituals - such as crafting a singular magical weapon for one's clan. Dwarves who regularly ingest residuum may take months, rather than weeks, to acquire an addiction.
2. Fringe societies and those outside of the Empire do not use residuum- the eladrin cities and their mythals are not the source of addictions to residuum.
3. Paying for the Residuum Use status becomes trivial rapidly for a PC moving up into the late Heroic Tier. It is effectively paying for a 20 gp elixir that lasts a week (an elixir with a risk)
4. The only expensive element of these rules comes through in the In Withdrawal status. PCs who fall into this state risk death, or a large bill as they go on a residuum bender.
a. The increase in expenses by tier is a reflection of the tier-based pricing schemes of various rituals in the PHB. Is it perfect to maintain realism? No. Does it keep the money a real issue? Yes.
5. It is generally cheaper to pay to return to Addicted Status than it is to succumb to death and use a Raise Dead ritual.
6. Regular users of the Ritualist Feat are guaranteed to become addicted to residuum, and 99 in 100 NPC ritualists in the Empire are addicted. This feeds their willingness to cling to the political structures that provide residuum, feeds their willingness to perform rituals for PCs with little overhead if they come with residuum, and also gives Ritualist players an additional motivation / complication.
7. Given the widespread use of residuum within the Empire, finding traditional components equates to adventuring. Going to a distant hamlet with a ritualist may give a player a small amount of traditional arcane, divine, or natural components, but more often it will result in information that leads players out into the setting to gather it by hand.
