Much like Deathbringer, Shadowdark is a variation on D&D 5E designed for players looking for a simple fast playing game. My problem with Deathbringer is it played very flat, meaning there did not seem to be any depth of play, much like the original version of D&D. A fighter was a fighter was a fighter, sure you could play them slightly differently and use different weapons, but at the end of the day, there was no real variance. The author of Deathbringer is working on an expanded version, but I suspect this is a low priority for him and so it may be years before we see a real product.
While Deathbringer could be printed out on a single sheet of paper, Shadowdark is a full blown RPG with no dependencies on any other games. The Shadowdark book contains everything you need, ancestries, classes, spells, magic items, monsters, the whole 9 yards. So right off the bat, Shadowdark is the better game. The Quickstart rules for Shadowdark can be had at;
https://www.drivethrurpg.com/product/413713/Shadowdark-RPG-Quickstart-Set
The Shadowdark core rules contain 6 Ancestries, Dwarf, Elf, Goblin, Half-Orc, Halfling and Human. This sounds all good and well, pretty standard fantasy RPG races, except for one small thing. None of them have darkvision. In fact darkvision is not a thing in this game. The only defense against darkness is natural light, torches or lanterns. In either case, they only last 1 hour of real time, not game time, real time, and once the lights go out, no one can see anything. That is except for the monsters, who are all dark adapted and can see in the dark just fine. This builds an interesting resource management aspect to the game, because the characters can only carry so many torches or so many flasks of oil for the lantern. This means the players have to plan out their excursions. They cannot take a long rest after every combat, because a long rest costs the characters 8 torches, or they have to build an actual fire, which has its own problems, especially in dungeons where there is little or no air circulation.
The game has 4 base classes, Fighter, Priest, Thief and Wizard. There are plenty more available online and creating new classes is pretty easy, but honestly, I am not terrible sure that is necessary. I have always felt the 4 core classes are pretty much all that is needed. Although, something I would probably house is multi classing, because it does make character building more interesting and allows the players to make characters more like the movies and books. One of the more interesting aspects of the game is, as the characters level, rather than getting set in stone powers and abilities, they get a 2d6 roll on a talent table. Rolling a 2 gives you an extra powerful talent, rolling a 12 lets you choose any talent off the table or gives you a +2 to distribute to you attributes as you see fit. 3-6, 7-9 and 10-11, give you a mixed bag of talents. The player will only ever get a maximum of 5 rolls off the table, assuming the game goes to 10th level, enough to get an interesting mix I think.
Another interesting aspect of the game is no saving rolls of any kind. All spells are roll to cast, which means every time the character casts a spell, the player must make a spell casting check against a DC of 10 + Level of the spell. If they succeed, the spell goes off normally, if they fail, nothing happens and they cannot cast the spell again until they have completed a long rest. There are no spell points or spell slots, the character can cast the spell over and over, until they fail a roll. This helps keep the focus on lower level spells, because they are easy to cast and players will tend to save the high level spells for when they absolutely need them instead of wasting it on three kobolds because they think its funny.
Overall, I think this is a solid game. If you have played any of the modern itinerations of D&D, this will be familiar to you. However it has been stripped down for speed and ease of play. Combat should not be bogged down with endless choices and because the torch is burning and no one wants the torch going out in the middle of a fight, players are motivated to expedite their turns. I am giving this one an 8 out of 10. I really want to play it.