This is a community challenge and should unlock once the time finishes if the goal is met.
You unlock the Imps very early in the game as apart of the first Blackbeard quest.
Next buy the detonate upgrade at a vending machine for the Imps. It costs ten soul orbs.
Now pick a spot and just continue to summon and detonate them.
The game has more upgrades you can get such as one that increases the amount of imps that you summon which will give you more progress much faster.
If 1000 of us detonated 250-ish Imps, we will hit the goal but I assume a lot less will be grinding this so you should do a lot more, but do whatever you can to contribute.
You should get this achievement while grinding: