I've run the Skyrim Dark Brotherhood line more times than I'd admit, and it still surprises me how many people don't know you can refuse it outright and wipe the Sanctuary off the map instead. So here's the whole arc, both directions, with the bits that actually trip people up.
Getting in (and the choice most people miss)
It starts with a rumour. Talk to enough Skyrim townsfolk and you'll hear about Aventus Aretino, a boy in Windhelm doing the Black Sacrament to summon the Brotherhood. Walk into the Aretino Residence, find him kneeling over a heart and bones, and he hands you a contract: go kill Grelod the Kind, the cruel matron of Riften's Honorhall Orphanage. Grelod's a free kill with no bounty, and frankly the kids throw a party afterward.
Sleep anywhere and you wake up in an abandoned shack, kidnapped by Astrid. Three hostages are kneeling and bagged, and she tells you one of them "contracted" your Grelod kill, so you owe the Brotherhood a life. Here's the fork. You can kill any one of them (or all three) to settle the debt and join. Or, and this is the part people sleep on, you can refuse, draw on Astrid herself, and kill her. Do that and the whole questline reroutes into "Destroy the Dark Brotherhood," which I'll get to.
The main questline, in order
Assuming you join, Astrid sends you to the Falkreath Sanctuary, password "Silence, my brother." From there it rolls through the contracts: Nazir's side-jobs for coin, then the big Penitus Oculatus thread. You knock off targets for the Aretino-style clients, but the spine of the story is a plot to assassinate the Emperor, Titus Mead II.
Key beats: the wedding contract on Vittoria Vici in Solitude (drop the stone balcony on her or just stab her. Chaos either way), framing and killing Gaius Maro along the festival road to discredit the security detail, then the dinner at the Emperor's Tower where you poison Gourmet's Potage le Magnifique and kill the chef Anton Virane to cover it. That gets you face to face with what you think is the Emperor. Except it's a decoy, Commander Maro's trap. You come home to a burned Sanctuary and most of your family dead.
The back half is grief and revenge: regroup at the Dawnstar Sanctuary with the Night Mother and Cicero, learn the truth about Astrid's betrayal, take the real contract on the Emperor aboard his ship the Katariah, and decide whether to take his side-offer to kill Amaud Motierre, the man who ordered it all. Finish it and you become Listener, the one the Night Mother actually speaks to.
A few things worth knowing: keep Cicero alive when the Sanctuary's attacked and you can recruit him later as a follower, which I'd recommend. He's a genuinely strong companion. And the Blade of Woe you pull off Astrid is a keeper.
Destroying the Brotherhood instead
If you killed Astrid in the shack, you head to Dragon Bridge, report to Commander Maro at the Penitus Oculatus outpost, and he sends you to clear out the Falkreath Sanctuary. You fight through and kill everyone inside, Arnbjorn included. That's it. The faction's gone, no Listener, no Night Mother, no Blade of Woe (well, Astrid's already dead so you got it early). It's a short, brutal alternative and the only way to permanently lock yourself out of the questline. Worth doing once on a paladin-type character just to feel it.
So which arc lands harder?
Skyrim's version is a slow-burn tragedy. A dying family, a betrayal from within, the Night Mother whispering you into a leadership nobody else can hold. Oblivion's is tighter and nastier: the "Whodunit?" party-house contract, the purification where you're ordered to murder your own Sanctuary one by one, and the Lucien Lachance traitor twist that still gets me. Oblivion leans into dread; Skyrim leans into loss.
If you want the lore threads, UESP's pages on the Night Mother and the Five Tenets are solid background reading.
I lean Oblivion by a hair, purely for the purification gut-punch. Which Brotherhood arc hit you harder, and did you ever actually take the destroy-it path in Skyrim?