This mod originally started as a sort of "Dark Brotherhood Requirements." I noticed there was nothing in the vein of "Thieves Guild Requirements" for this faction, and so I set out to make something simple, where you just needed to be sneaky about killing Grelod. However, while researching the Dark Brotherhood, it hit me how ridiculous it is that any time you cross the guild, you can just pay them 500 gold, and that was that. And so, with these ideas percolating in my mind I came up with the following features:

• The Dark Brotherhood won't take just anybody. To get it, it is not enough to kill Grelod, but you must do so using either a) sneak attack(s), b) frenzy spells, or c) reverse-pickpocketing poisons. In essence, if at any point you hit Grelod with a non-sneak attack or spell, it's game over.
• Also, just to be really safe and Requirements-y, you need 30 Sneak to get in. Also, just for fun, Grelod has 15 health, because 1 just looks so artificial and hand-holdy.
• When the Dark Brotherhood passes on you, however, that is not the end of the story. For you did still take one of their kills, and for this you must be punished. Therefore, when sleeping after receiving the note in this scenario, you will be attacked by Astrid instead of kidnapped. Assuming you kill her, this begins the "Destroy the Dark Brotherhood" questline.
• Once you join the organization, you can't let down your guard. In order to avoid incurring the wrath of the guild (paying Nazir), you have to kill your targets without incurring any crime for it. On the technical side of things, this means that your violent crime gold against the target's crime faction must not increase between the first time they are hit, and the time they are killed.
• The only target I did not do this to was the Emperor, since you're already on a boat.
• The guild now employs a "three strikes" policy. If you offend the guild thrice, whether through the vanilla means or committing crimes, you will be permanently ejected from the organization. Basically, the state where you can't accept contracts or talk to people during "Honor Thy Family" becomes permanent, and all quests you have running for the DB are marked as completed and stopped. Nazir's dialogue is also filtered so that you cannot pay him if you are kicked out, just to be safe. Lastly, the player is of course internally removed from the Dark Brotherhood faction.
• There is an exception to the ejection policy. If you have become the Listener of the Dark Brotherhood, you cannot be ejected, for obvious reasons. However, gaining crime gold while assassinating the recurrent targets still yields the (reduced) fine, because it's like Nazir says, you're setting a bad example.
• Finally, this mod uses no dialogue, because I hate silent dialogue. As you see in the images, everything is handled with Message Boxes.
  

Is this mod "lore-friendly"? Well, I was inspired by the second of the Five Tenets:  "Never betray the Dark Brotherhood or its secrets. To do so is to invoke the Wrath of Sithis." I would argue that by being seen by guards to commit murder, they might start thinking that you are up to something, and so eventually be lead back to the Brotherhood as a whole. thereby revealing its "secrets." However, ultimately, this is supposed to be a Gameplay-enhancing mod, giving you a reason to play as an actual assassin while assassinating people. 

COMPATIBILITY
This mod should be compatible with any mod which does not touch the same things as it. Since the scripts are loose and the esp can be easily analyzed in xEdit or zEdit, that should be trivial to figure out. Alas, this mod does touch some NPCs, but they're not too important, and you can make all the patches you want. As a particular note, this mod should be compatible with Your Choices Matter from below, although I have not tested it at all yet.

I designed this mod as a compliment to other mods I use giving the other factions actual requirements and whatnot. Since I have a whole folder of just faction-themed mods, I'll share what I currently run:
-this mod (duh)
-Your Choices Matter - A Dark Brotherhood Expansion SSE
-Thieves Guild Vault
-Thieves Guild Requirements
-Better College Application
-College of Winterhold Entry Requirements
-Obscure's College of Winterhold
-Immersive College NPCs
-Enhanced Skyrim Factions - The Companions Guild (personally ported from Oldrim)
-Civil War Overhaul Redux
-Bards Reborn Student of Song Become a Bard Expansion with Bard Spells
(yes some of those are aesthetic but they're still for a specific faction)
I cannot be bothered to go get links for them all, go look them up yourself if interested.

OFFICIAL PATCHES
I currently have a patch for "Innocent Lost Alternative." It merges sub-records of both mods for Grelod, and it also makes so that if you call the guard, the Dark Brotherhood will grow angry, reject you, and launch an attack, just as if you had non-sneak attacked Grelod yourself.
I also (as of 6/22/2021) now have a patch for "Filling in the Blanks - Unique Faces for Bandits and More." I basically just copied over my script additions to the 10 radiant quest targets, so that they now have both my scripts and this mod's new faces. This patch should work with either the Vanilla or EFM version, but please let me know if something is amiss.

CREDITS
A very large amount of thanks goes to Apoapse, who put up will all my questions as I learned Papyrus to make this mod.

Thanks to Arthmoor and everyone else who makes USSEEP, which this mod requires and forwards script and NPC fixes from. Thanks to the creators of Even Better Quest Objectives, whose more descriptive descriptions I made a point of forwarding in this mod.

Thanks to all the people who worked on the mods which this mod has patches for.