I've thought of an alien race that are masters of warfare. What makes them so dangerous is their inability to feel pain. But now I'm wondering. Is this possible? Bonus points for their biology.
Being unable to feel pain is a real human condition called CIPA. However, it is very rare and can be dangerous as you have no sense of doing your body harm. A common example given is breaking a bone. Most people would be in agony, but someone without being able to feel pain, wouldn't know unless they saw that the bone was broken and sticking out of their leg. I found this simple explanation on how CIPA works:
It doesn't disable the children in any way; it simply interrupts the growth of the tiny nerves that communicate heat and cold sensations to the brain.
This case study shows some of the psychological effects in humans of not feeling pain, and how it has an effect on everyday life.
Now if an entire race has the inability to feel pain, then the effects on life might be different. For starters, they'd find it much harder to become an intelligent species as they'd have a lot of trouble keeping the species going as internal illnesses are difficult to detect without any pain. Many terminal illnesses such as cancers list pain in specific areas as a sign of the disease. However, you could avoid explain how they managed to evolve into an intelligent life form by saying they were created by some great power.
If the races main focus is war, then they would have to go to war with a species that feels pain to realise they have an advantage over the enemy. Until the reptiles come into contact with species that do feel pain, they would have no understanding of the concept. Once they realise they have this exclusive advantage, then they would develop warfare tactics that work off of their inability to feel pain. If the species is truly dedicated to war, then perhaps the people feel honor for dying in battle, so some will run forward as arrow fodder willingly, or be happy with their brethren climbing on top of them to get over a wall. After all, if you're at the bottom of the stack and you can't feel pain, you won't notice the strain on your muscles from supporting the weight.
In this How Stuff Works guide they explain that children are often taught to treat seeing their blood as a warning sign, and to seek help because they don't understand that they are hurt. These reptiles would probably have a similar system in place, but may have a high infant mortality rate simply because the child has no sense of fear. A lot of their medicine and culture would be based on detecting illnesses without pain as an immediate guidance and instilling a sense of fear of pointy objects into children to try to lower the rate of accidental and preventable deaths.
I feel like not being able to feel pain at all would actually not be an advantage... but I would suggest, alternatively, that a race of deadly warriors would feel pain, but would respond to it differently.
As humans, when something causes us pain, we tend to avoid it. During our evolution, things like predators, thorns, falling from high places, etc. caused us pain, and these are all good things to avoid entirely.
But you could give your warrior race a much different evolutionary history.
Take for example, birds of paradise. They look ridiculous, because of a very long history of choosy mates. They evolved to look more and more outrageous, as the most outrageous looking ones had the most reproductive success. This shows how selective mating practices can transform a species.
Now take your alien warrior race; say they have a long history of fighting over mates. If winning a fight means you get to reproduce, then natural selection would favor those who are not only strong/fast/dangerous/whatever but also those who can take some damage and not give up. If winning a mate fight means instant reproduction, then even eventually lethal injuries could plausibly not slow them down. Given this evolutionary history, the feeling of pain might actually excite them. In battle, the feeling of pain would just make them fight harder.
Add in a culture that idolizes the best fighters, and glorifies violence, and I think you've got a pretty scary race.
I dunno if this is what you're looking for or not, I kinda changed it on you, but it seems scientifically plausible to me, there may even be species here on earth that are like this. I don't know. Either way, hope it helps!