All Mobs Hate The SUN

Hostile mobs catch fire when the sun is up and lose three hearts of health every few seconds if they are not in water or immune to fire. This effect only applies to mobs that can be burned (most hostile mobs) and does not affect passive creatures.

The damage happens automatically on the server, so you will see mobs flicker with fire and their health drop while the sun shines.

Install the mod on a Forge 1.12.2 server or world to see the change.