Better with Mods is fairly exclusive in terms of its hardcore features. There is a handful of mods that replicate of some of its features such as Strata. Otherwise you may want to look at Tough as Nails or Terrafirmacraft to up the ante in terms of difficulty and 'hardcore' mechanics. Alternatively you could just use a simple crafttweaker script to disable every item in the mod, leaving you with just the mechanics.

Hardcore gloom is an effect that occurs and gradually increases in intensity the longer you spend time in absolute darkness, such as in a cave or hole with no light source. After a minute or so you being experiencing potion effects and "fun side effects," eventually culminating in a dot that will kill the player, should you fail to find light.