A Greek setting campaign focusing on a lot of exploring myth, and combat with monsters.
Races and classes are very restricted, most non core races are not allowed and many setting based classes are just as banned. Paladins will be changed heavily in the flavor department and monks are near non existent. Clerics receive some additional skills and a weapon proficiency based on deity and some spells work a bit differently.
The mythology is BASED on the Greek mythos and legends. they are meshed with some other stuff that simply make DnD work. combat will be paced more like it is in “traditional” DnD, were it goes minions, guards, the big bad boss.
The Nightmare Child campaign will focus on a lot more on Dungeon crawls, and finding obscure lore. Lovecraftian style means a lot of deep dark places and sometimes magic just wont save you. most of the dungeons will be some what short and just involve a trek underground, and a few floors of the actual dungeon. or just as often it will be messing around in an old library or a lonly tower on the side of a mountain. finding out exactly what that monster is will be far more difficult and important then in most settings.