This material was inspired by Dylan Clark's horror short film "Hatched" (2021). You can watch it at the Terror Titans blog by clicking here. (Ideally, you came here from that post, as even the title of this one may be a little bit of a spoiler.)
The game mechanics attached are for the OGL Modern variation of the d20 System, but it should be easily adaptable to most other RPGs that are out there.
DOPPLEGANGER EGGS
Doppelganger Eggs are one-shot magic items that were developed in ancient times by Atlantean Biomancers, and the art of creating them is now only known to the secretive Witchkind and a tribe of equally secretive doppelgangers (creatures capable of assuming the form of any humanoid creature they kill, as well as absorbing their most ingrained personality traits and skills) who work with them. By performing a complex ritual, a doppelganger is put into the egg. When the egg is cracked or hatches when the magic wears off, the doppelganger swiftly returns to his original size. It can then infiltrate the household into which the egg was smuggled (usually among other food items). The Atlanteans would use the eggs to unleash assassins and infiltrators on unsuspecting enemies, and that is how they are used in modern-times as well.
Creating Doppelganger Eggs
Doppelganger Eggs are created using any eggs laid by a non-magical bird or reptile and a willing doppelganger. Two beings with knowledge of how to create Doppelganger Eggs and the ability to cast 5th-level spells and knowledge of the Transmutation and Necromantic magic schools must work together to create the Doppelganger Egg. Usually, the two spellcasters and the Doppelganger are long-time associates, if not actually friends, so all those involved trust each other to do the best possible job and to take care of each other if a client/customer tries to betray them or welch on a deal.
The ritual must be performed in a specially constructed location with a permanent enchantment area used exclusively for the creation of Doppelganger Eggs and a small number of related magic items. Over the course of three hours of intensive spellcraft, a doppelganger and an egg are merge. By the end of the process, the egg is undetectable from similar eggs, except that it radiates a faint aura of necromantic magic if such is detected for. Up to four Doppelganger Eggs can be created at one time, and the ritual is prolonged by one hour for each Egg in addition to the first one. The makers of the Eggs can perform their ritual once per day, as it drains them to the point where they must have a full night rest before they can cast magic or perform rituals again.
Doppelgangers that work with the Witchkind conform to the standard statistics in the rule book.
Acquiring Doppelganger Eggs
To acquire one or or more Doppelganger Egg, a character must first gain the trust of the Witchkind, and then they must be able to locate a very secretive group of artificers in their society since what they do is not exactly looked upon kindly by any authorities.
Each Doppelganger is made to order. Upon making contact, those interested in gaining one or more Egg must provide a general description of what they want to achieve with the Egg, so the Doppelganger knows who to kill and/or replace and what to do once that has been done. A mission can be a straight-forward assassination or it can be a long-term infiltration. For a relatively simple mission, the cost to acquire an Egg requires a DC18 Wealth check (with the check increasing in difficulty by +4 for each additional Egg being ordered) with the base Wealth check being perhaps as high as DC28 for a complicated, long-term intelligence gathering mission). The price can be raised or lowered if the purchaser wants to impose specific restrictions or demands upon the Doppelgangers that aren't directly related to the mission at had (such as demanding that the location and/or assumed identity must be vacated as soon as the mission is complete). The price also increases if the purchaser wants the Witchkind and the Dopplegangers to handle the placement of the Eggs.
Using Doppelganger Eggs
Doppelganger Eggs are usually placed among innocent groceries that are brought into a household or other place where a target lives. As soon as the Egg is cracked or 48 hours pass (the duration of the magic), the Doppelganger bursts forth and swiftly returns to its full size. Any characters who witness this even must roll successful Wisdom checks (DC14) or be so startled that they will not be able to take action that round the Doppelganger appears, and the Doppelganger automatically gains initiative the following round.
The Doppelganger suffers a -2 penalty to all skill checks, saving throws, and attack rolls for the first six rounds after emerging from the Egg, due to the physical and mental shock of recovery. As such, it will initially try to retreat if it isn't catching potential victims completely unaware.