To all iOS and Android players. Please note that the information provided in this wiki will not be 100% accurate to your version of the game. The wiki is on-par with v2.3, while the mobile version is identical to v1.7.
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.
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.