At first I said there would be no classes native to Grot, and all players would hail from Beyond. Well, I changed my mind. Here are two character class ideas for Grot. I may think of more, but probably not.
Zhuvimbie - You are a bit more than dead. You are un-dead. You might have been a human or something else, but now you're dead. Un-dead. You remember it all, too. Cleric is not an option. Wizard, fighter, thief. You might have been a cleric before, but now you're dead. Un-dead. You don't need to breath or eat, yet you sometimes crave brains, possibly for special reasons. You can be unnervingly frightened by clerics, and may be pained by holy water. Other details, as appropriate.
Minor Demon - Yep, a demon. You might also be a daemon, but probably not a devil. Not so much Bible Demon as Realm of Chaos Demon. Depending on where you visit, certain sorcerers in-the-know may be able to contain your movements within pentacles. Similarly, clerics and holy water may be an annoyance. On the plus side, you get 2 Neat Things at first level. Either a Nasty Power or a Horrific Mutation. One of each or two of a kind, your choice. Every level you get 1 more Neat Thing, and you get to pick which table you roll on. Nasty Power or Horrific Mutation.