Re'tu
The Re'tu (sometimes spelt "Re'tou", "Reetou" or similar) are a fictional alien race from the sci-fi television show, Stargate SG-1. Template:Spoiler
The Re'tu are a race whose molecules exist 180 degrees out of phase from our reality and were first introduced in the second-season episode "Show and Tell." They cannot be seen by humans and can only be interacted with physically to a limited degree; however, they can see humans just fine. They can become visible when exposed to radiation emitted from a Transphase Eradication Rod device (or TER). When exposed, Re'tu appear as shadowy aliens with a humanoid torso, spider-like lower body, and insect-like head with two bug eyes on stalks.
Re'tu technology is fairly advanced; at the very least, they are advanced enough to use energy weapons and able to create genetically engineered humans (namely the child ambassador nicknamed Charlie though his body was defective in several ways).
Because of their invisible nature, the Re'tu were deemed a threat by the Goa'uld, who attempted to exterminate them. Re'tu also emit a "vibe" that causes severe pain in a Goa'uld symbiote, which allows Goa'uld and Jaffa to detect their presence. The aggression of the Goa'uld has split the Re'tu into two opposing factors: the majority Re'tu government who are non-hostile towards humans, and a large splinter group of Re'tu rebels who believe that the only way to fight the Goa'uld is to destroy all their potential hosts, including all human beings living in the galaxy. The rebels accomplish this with tactics aimed at killing as many humans as possible.
Re'tu fight with powerful arm-mounted energy cannons that burn large holes through flesh and metal. Because they are partially out of phase, Re'tu are highly resistant to physical weapons, which only "slow them down" (While not expanded upon, it is logical to assume that this means that physical weapons cause damage, but not to equal to the damage done to matter on our phase. Re'tu can thus be killed by physical weaponry, but only after having been subjected to far more shots than would kill a similarly sized same-phase creature). Re'tu are quite vulnerable to their own weapons as well as the energy beams fired by a TER device, and a single hit from either will cause them to explode messily.
The Re'tu only actually appear in one episode, but they are mentioned several times throughout the show. Whenever something bad happens in plain view yet no visible cause is apparent, the invisible Re'tu are usually amongst the prime suspects. In a rumored upcoming episode of season 10, an invisible enemy sweaps through the base and attacks several people. It is possible that this intruder will be a Re'tu
