Tempest Achievement in Dishonored
You killed 6 enemies in less than 1 second
How to unlock Tempest
Required Power: Bend Time 1 (Bend Time 2 suggested)
You will need this power to gain this achievement. This will require you find 10 runes to have it upgraded to 2. You must have 6 enemies on screen and they must all basically die at once. There are likely several ways to kill them and different scenarios that it works, but below is the best method I found while playing the game.
Below in the video, there is perfect spot to perform this action. This is of course if you have a High Chaos level because more weepers appear. During Mission 5, you will be on your way to the mansion for a party. After crossing the bridge where the tallboy enemy is, take a right and then a left down the street. You will come to building that has a bunch of stairs leading up. At the bottom on this floor are around 6 or more weepers bunched up. Kill them while in Bend Time (using 2 grenades worked like a charm).