Nerves is a very flexible bow that can be charged before firing. The more you charge it, the harder it will hit. But don't think that it takes a long time to charge it -- only in 0.5 seconds it will be fully charged and deal critical damage.
The blueprint can be found at The Ramparts and only at a certain time. Go to the far right of the location and jump onto an invisible platform, which will appear only if the last tower doesn't open the exit to the next stage yet.