The Dark Ritual follows the Pacific Old School rules with the following exceptions:
Deck Construction
Your deck may only contain basic swamps, non-basic lands, artifacts, and mono-black cards originally printed in Alpha, Beta, Arabian Nights, Antiquities, Legends, The Dark, and Fallen Empires. Additionally, you may play no more than 4 artifact cards in your deck.
There are no restricted cards, only banned cards, which are: all ante cards, Mind Twist, and Hymn to Tourach. Yes, that means you can play 4 Black Lotus or 4 Demonic Tutor, if you really want.
Here's a Scryfall search link that shows you all of the legal cards: Legal Cards
Reprints with original frame and original art are fine, e.g. your 4th Edition Royal Assassin is fine, but your 7th Edition Drudge Skeletons are not.
There are no sideboards in The Dark Ritual.
Gameplay
Matches of The Dark Ritual usually consist of only a single game. You must play unsleeved, without a playmat. You must also play outdoors at night near a fire (preferrably a bonfire).
Cards named Swamp have errata (see below). The tournament organizer will provide an Ebon Stronghold card whenever you use the added ability that physically destroys your Swamp. This is to maintain a legal deck throughout the event and slowly lower the number of Swamps available to you as the games go on.
Errata for Swamp
Basic Land – Swamp
Tap: Add {B}
Tap, Cast this card into the bonfire: Add {B}{B}{B} and put an ‘Ebon Stronghold’ card from outside the game into your graveyard.
Errata for Ring of Ma'rûf
Ring of Ma’rûf
{5}
Artifact
{5}, Tap, Exile Ring of Ma’rûf: The next time you would draw a card this turn, instead choose a colorless or mono-black card you own from outside the game, and put it into your hand.
Note: this maybe be used to get any colorless or mono-black card you own from outside the game that would otherwise be legal for the game in which Ring of Ma’rûf is activated. You can get your 5th Black Lotus or Demonic Tutor, for example.