Stun Grenade

The Stun Grenade is a grenade skill which stuns nearby enemies on detonation.

Location
The blueprint for Stun Grenade is located in the Ramparts, in a secret area in the very last, far-right tower. The tower itself doesn't always spawn, so it may require a few runs to get it, but if it does, it will always be the last tower.

Trivia

 * Previously named Flashbang.
 * The Stun Grenade is indeed designed after a real-life flashbang, having a similar general design and the ability to disable enemies.