Well a few more Torchwood episodes have passed and the finale'll be rolling along before we know it so let's get on and catalogue a species or two shall we?
Continuing a long list of "it's an alien" explanations for all things unusual in the Doctor Who storyline, the Durac nonetheless made for an interesting character and was well used in the episode. And unlike many other episodes the transition from superstition or religion into extra-terrestrial life actually fitted and made sense. It's image was quite good too, not just a grim reaper copy and not wildly dissimilar either. The connection to the Weevils was good, especially with Owen's previous connection to them, it would have been nice to see more of that explained, but I guess there's only so much you can know while fighting death!
Durac is a powerful being that transcends the physical realm we occupy and is kept from entering it unless someone willingly or unwittingly lets it enter. It seems to be connected to a "resurrection glove" one of two found by Torchwood which had the capability to raise people from the dead for varying lengths of time and with varying effects. The first glove raised people from the dead for a brief moment by taking away life energy from the wearer - but when dead Torchwood member Suzi Costello used it to bring herself back from the dead, but killing Gwen in the process the glove was destroyed. When Owen Harper was killed, Jack hunted down another glove which had vastly different effects. Firstly, while Owen's mind and body functions became active, his body remained dead with no pulse or respiration. The glove also awakened Durac, who first posessed Owen's mind then entered the world as a semi-corporeal cloud, featuring a skeletal body. It needed thirteen souls to fully enter the physical world, but was stopped by Torchwood. They read ancient documents concerning what is now known as the Grim Reaper who entered Cardiff after a girl was untimely resurrected, but was defeated by that very same girl because the Durac could not take her life. Owen used this principle and his similar state of body to combat the durac and repulse it from our world. The Durac seems to have some connection to the Weevils who emerged in large numbers when it entered and who worshiped it as a deity even while it still posessed Owen, this connection remains unclear but as Weevils have been on Earth for a long time they may know more of his original entry, or he may even have influence on their homeworld.
It is unknown whether this Tarot reading child is an extra terrestrial, but she is worth mentioning as Jack noted that she should not be taken as she appears and that she has been doing her work for a long time.
Shapeshifters are as old as Sci-Fi TV itself, but again this principle was used well and to good effect by the Torchwood production team. I think an intelligent step was not to show the transformation process which added to the suspense and avoided potentially cheesy CG morphing. The species were scary, but subtly so and struck a balance between wanting to survive, but also pleasure from killing and destruction. The concept of impregnating Gwen was very funny, but I think killing an alien's child, no matter how violent the parents shouldn't be something taken lightly, but unfortunately that scene lacked that depth and didn't quite make up for it in energy either - barring that it was a good episode and a good species.
Nostrovites are a race of carnivorous humanoids with a most unusual breeding habit and a host of other talents. They posess a proteus gland, which enables them to change form to something of similar shape and mass, such as a human. Naturally they appear to be reptilian, though evidence suggests they may be warm blooded. Upon examination of a dead subject Dr Owen Harper suggested their shapeshifting may have come about because of how ugly they are! Their shapeshifting is suprisingly accurate, until they are enraged or out to hunt, when their natural red eyes and pointed teeth show through. They seem to need DNA samples to complete their transformation, which actually makes for a more stable process, their preferred method of obtaining these samples is by biting a chunk out of a victim, though it seems more subtle methods are just as accurate. They also seem to posess the ability to create an adhesive webbing to pin a victim to keep for later. It is, however, another of their properties that brought them to the attention of Torchwood. While hunting a male Nostrovite in Cardiff Gwen Cooper was bitten before the target was shot down by Jack Harkness. In the morning she found herself impregnated to the effect of looking several months pregnant in human terms. Torchwood discovered that Nostrovites hunt in pairs, male and female, and the the female would come to rip her newborn from the human host. Naturally Torchwood intervened, and the Nostrovites were taken down despite a suprising resilience to small arms fire.
And that's it, one more to go for this series and then Doctor Who looks set to reappear before too long.