Over the years my various gaming groups have had a laugh at the idea of fighting giant centipedes. I have to admit, that as a DM, the never really struck fear in my heart either. Until I rolled over one in my sleep last night and it stung me.
I woke up screaming. While I have never given birth to a child, or been shot, I have enough data points to be a fair judge of pain. I have had non-trivial invasive surgery, a really bad car accident, a serious electrical shock and a gushing head wound. This ranked up there with waking up after the accident. My right leg, from the mid thigh down, was paralyzed for several minutes. It was painful to stand on for over an hour. It is still sore and stiff today. All of that came from a sting wound the size of a mosquito bite.
The doctor tells me that th potency of the centipede poison is proportional to the size of the centipede. There is no way to know how big this centipede was, but I live in a basement apartment and I see them running around all the time. They tend to be between 2 and 4 inches in length. Imagine what one that is several feet long could do to your character. I can guarantee that from now on, the idea of a giant centipede is no longer laughable to me.
Think about them crawling around as you go to sleep tonight, I know I will be.