It is a predatory shrimp that belongs only in a species tank. They will kill off the life in your tank. They have club like claws that they smash open shells with. They are quick, and somewhat clever (for a shrimp). You don't want your hand near it, because they have a reputation of being able to actually break bones. There are ways to get them out, and people deal with them all the time. I did not get one on my LR, and most others do not either. It does happen, but don't be afraid to get LR because of that.
There are a lot of other things that come on LR that are really fun to watch. Seeing new life appear from the rock was a lot of fun for me. I got a lot of mini brittles, fan worms, snails, and the like from mine.
If you have a stocked tank all ready, some advice... let the LR sit in a bucket of tank water for a few days with a pump, to allow for some die off. You will always have a little die off in transporting, but there may be a little more for shipped LR. If it was cured and picked up from your LFS, I'd say put it right in.