This is, unpleasantly enough, based on an actual marine parasite that attacks fish. I just modified it to fit land based creatures.
The Tongue Biter lives in cracks in the stone and other small spaces in underground environments. After it is born, it can live for about a week before it dies of hunger. Before this time, it has to find a host to live off. When it does, it creeps into this sleeping creature's mouth, injects a powerful anesthetic into it's tongue, which bodypart it then devours. After having eaten the whole tongue (this takes about six hours), it hooks on to the stump, eventually grafting itself on, in essence becoming part of the hosts body.
Once latched on, it starts sucking the blood of it's victim for nourishment, and starts to produce young. These young crawl out of the hosts mouth at an amount of 1d20 twice a month. The Tongue Biter is very hard to get rid of, as it connects itself to the hosts nervous system after about 24 hours, and at that time, any pain felt by the parasite is felt by the host as if it was part of it's own body. It requires no attack rolls to kill, but may require other rolls to tolerate the pain, at the DM's discretion. Once off, the spell Regenerate is required to grow back the victim's tongue.
While the Tongue Biter lives within an adventurer's tonuge, it does no actual damage, but causes a number of inconveniences, the chief of which is an inability to speak. While the he will be able to eat, the character can not experience any sense of taste, which may carry some implications. For example, depression and loss of weight may plague the adventurer, common side effects losing one's sense of taste. In other unfortunate side effects of not having a tongue will apply as the DM sees fit.