I have found in 2 different places, first is near very large rock to the east of the Bridge keep in the dry riverbed. The other spawn is down the riverbed right past where it forks, take the right path if you are running south down the river. The PH seem to be rock writhers, the other writhers don't seem to spawn anything. Spawn time seems random, probably under 30 mins usually. They are a pain, it took me about 3 nights to kill 6 of them.