Tempest is an achievement/trophy in Dishonored.
How to Acquire[]
In order to get this achievement Corvo Attano must kill 6 enemies in less than one second. He can upgrade Bend Time to Tier 2, use it in an area with a large number of guards and kill them as quickly as possible.
Alternatively, he can go to an area with many guards, create a distraction to lure all the guards to one spot and cook a grenade so that it explodes on impact.
The easiest way is to knock out five guards, put them in a pile, lure another guard over to the pile, and throw a grenade at them, killing the lot. This may require some quick timing as bodies (alive or dead) that are lingering in an area have a tendency to despawn if other NPCs are still about.