This is a shockingly awful product; one that basically does not function alongside 5e's mechanics at all. It rips out integral mechanics, replaces them with new ones (some are interesting in a vacuum, but none slot into 5e's systems very well), and then offers no guidance whatsoever for making the resulting frankensteined-together mess of a game actually function.

As an example: The rules offhandedly mention you should just remove bonus actions entirely. Now, bonus actions are a core mechanic of 5e. Every single class has at least one mechanic that uses them. So what does this mean for, say, the Rogue who can disengage as a bonus action? Or the Barbarian who needs a bonus action to enter their rage? Well, the book doesn't tell you. It just assumes these things will, somehow, still function at all when bonus actions are unceremoniously removed from existence. This is a ten-year-old's understanding of game design. It's unworkable.

Other things the game suggests you remove: Initiative (what happens to characters with bonuses to initiative? Who knows), measured distance and movement on a grid (so now abilities that increase your movement speed or reduce enemy speed do nothing), and spell slots. These are not optional mechanics in 5e, they're core to the system, and everything is built around them.

These rules want to add danger and tension, much of the time by cramming in rules from old editions with no understanding of why those things worked in their respective systems (like class-specific XP progression, for example), but they entirely fail to accomplish that goal in any meaningful way. Some rules directly reduce lethality, like a videogame-style save point you can go back to (???) and monsters randomly rolling to decide what attacks they use, instead of doing what is most tactically advantageous. The few rules that do interact with the game's numbers feel like the designer just eyeballed them with no understanding of 5e's math. It's a failure of design on multiple levels, and at best, maybe some of these rules could serve as vague inspiration for actually functional homebrew with effort put into it.

The most confusing thing is that this comes from the same designer as Index Card RPG, which is actually quite good, with a decent understanding of the effect mechanics have on play; the fact that ICRPG and this unusable mess were made by the same designer baffles me. If I'd made a product as popular and well-designed as ICRPG, having 5e hardcore mode under my belt right next to it would be a stain on my reputation. Notably, the few good ideas this product has are mostly cribbed right from ICRPG, where they actually work, as opposed to here where they're a square peg in a round hole at best. Save your $3 for the good Runehammer product - this one is so bad I want my $3 back.