- A level 17 Wizard (School of Evocation) with an Intelligence score of 22 is able to create a finite but arbitrarily large number of simulacrums of himself, using the widely known simulacrum cheese strategy.
- The immediate utility of the additional spell slots has run its course for a large number of these simulacrum, and now there exists what basically amounts to a faction comprised of duplicates of this hyper-intelligent wizard, but who are each only able to use cantrips, ritual spells, and magic items.
- Due to the cosmology of the setting, simulacrum do not have souls but do have animating spirits.
For me, the logical "opportunities" for a used up simulacrum would be:
- Serve as a "think tank" comprised of friendly 22-Intelligence creatures that ultimately do lore research for the original wizard and periodically provide new clues or connections
- Serve as a Librarian and Scribe in various libraries around the world, copying books for the original wizard, ideally copying specifically information that the original wizard does not know (due to a complication described later, this could simply be copying anything they do not understand or comprehend into a notebook)
- Serve as a Postmasters, using their flying familiars to deliver letters. Taking this a step further, creating an "arcane newspaper" comprised of all of the major updates from all of the simulacrums from across the world.
- Serve as a spy network, keeping watch over various locations the original wizard cares deeply about, simply responding to periodic castings of Sending with status updates (which might just basically be a list of keywords describing each day of the month based on their observations over the last 25 days)
- Keep an eye out for spell scrolls, spellbooks, or useful spellcasting components, such as anything consumed by casting a spell, and buy them for the original wizard to collect later
- Educating a town or village by giving lectures of things it already knows. Future adventuring wizards might bond over having the same teacher teaching the same class at the same time in two different locations and chalk it up to magic.
- Mass Production, if the original wizard was proficient with artisan's tools, simulacrums could be a free source of skilled hirelings
- Manual labor. Every wizard's tower could always use additional fortifications.
The relevant parts of Simulacrum (from the Basic Rules, emphasis mine):
The duplicate is a creature, partially real ... and otherwise be affected as a normal creature. The simulacrum lacks the ability to learn or become more powerful, so it never increases its level or other abilities, nor can it regain expended spell slots.
The relevant parts from Using Ability Scores (from Basic Rules, emphasis mine):
Intelligence measures mental acuity, accuracy of recall, and the ability to reason. ... An Intelligence check comes into play when you need to draw on logic, education, memory, or deductive reasoning. ... When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object, discern from the appearance of a wound what kind of weapon dealt it, or determine the weakest point in a tunnel that could cause it to collapse. Poring through ancient scrolls in search of a hidden fragment of knowledge might also call for an Intelligence (Investigation) check.
Relevant parts of Speak With Dead (from Basic Rules, emphasis mine):
... The corpse knows only what it knew in life, including the languages it knew ... This spell doesn’t return the creature’s soul to its body, only its animating spirit. Thus, the corpse can’t learn new information, doesn’t comprehend anything that has happened since it died, and can’t speculate about future events.
I recognize that the way Curse of Strahd handles soulless creatures is not meant to be used in every setting, but I do want to include it here for the sake of completeness. The relevant parts about soulless creatures from Curse of Strahd (emphasis mine):
Barovians without souls are empty shells created by [Strahd] ... they tend to be bereft of charm and imagination and to be more compliant and depressed than the [Barovians with souls]. They dress in drab clothing, whereas Barovians who have souls wear clothes with a splash of color or individuality ... Barovians without souls are maudlin folk who experience fear but neither laugh nor cry
Similarly, for the sake of completeness I want to include the Kenku race. The relevant parts about a Kenku's soul from Volo's Guide to Everything (emphasis mine):
...the spark of creativity was torn from their souls ... As a result of their lack of creativity ... Flock leaders enforce discipline and minimize conflicts, but they fail at effective planning or crafting long-term schemes ... kenku have no ability to invent new ideas or create new things. Kenku can copy existing items with exceptional skill ... They can copy books, make replicas of objects, and otherwise thrive in situations where they can produce large numbers of identical items ... Although kenku can’t create new things, they have a talent for learning and memorizing details ... A kenku who learns of clever schemes and plans devised by other creatures can put them to use ... The kenku lack the talent to improvise or alter a plan, but a wise Master sets multiple plans in motion at once, confident that underlings can follow orders to the letter.
I think it follows that the "spark of creativity" being torn from Kenku would imply that the ability to invent is housed in a creature's soul, and therefore the same issues, or more severe issues, would apply to creatures that lack a soul.
The biggest issue for me is how to square "bereft of imagination" (from Curse of Strahd), "no ability to invent new ideas" (from kenku), "can’t learn new information" (from Speak With Dead), and "lacks the ability to learn" (from Simulacrum) with "make deductions based on those clues" (from Investigation). Making deductions seems like learning to me, or at the very least seems like a new idea. This seems to rule out some of the "opportunities" listed earlier for used up simulacrum, such as serving as a researcher or "think tank" (since they can't invent new ideas) or creating an "arcane newspaper" (since they can't create new things, and each version of the newspaper would be a new thing, and even if they could, the other simulacrums would not be able to learn the contents of the newspaper). I am more interested in answers that describe how a high Intelligence score affects a creature without a soul, and, as the DM for the campaign in which this situation exists, I am aware that I can arbitrarily change the game to my liking.
Relevant other questions: