Here's a handy tool for D&D DMs – Want to add a special attack to a creature? Convert a single target attack into one by cutting the damage in half. Turns single attacks into multiattacks or one target spells into area of effects.
— Mike Mearls (@mikemearls) August 6, 2018
Using the stock ogre as an example – give it a big flail that can also do an area attack by keeping that attack bonus the same, extending the targets to everything within 10 feet, and using 1d8+2 for the attack's damage.
— Mike Mearls (@mikemearls) August 6, 2018
If you want to get fancy, you can also do half damage if the attack roll is above a certain threshold, regardless of AC, to make it kind of saving throw-like
— Mike Mearls (@mikemearls) August 6, 2018