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It's easy to call this indie gem a mix of Metroidvania and Dark Souls. I'd however say it's really the former with inspiration taken from the latter. For a true mix it's missing the RPG elements used by the Souls games. Hollow Knight has no leveling and no equipment variety. You boost your silent character by collecting power ups, upgrading your main weapon and collecting charms that provide passive benefits. You lose your money upon death and have the ability to reclaim it if you make it back to your place of death, but it's strictly for purchasing stuff like the aforementioned and maps.
Hollow Knight's strongest point is its tight gameplay. With both melee and spell attacks at your disposal there's a breadth of both combat and platforming challenges to overcome. There are no consumables - in order to restore health you have to either rest at a checkpoint or use a spell that consumes MP. MP are recovered by striking enemies and said spell requires a moment of charging up to execute. It's easy to get punished for attempting to heal at the wrong time and if you're not playing offensively you're not able to collect the necessary resource to heal to begin with.
The game's world is really big, and yet each area manages to provide a unique experience with fun platforming and inventive enemy attack patterns. There's more bosses in the game than one might expect of an indie title, and they all offer a distinct experience - although a commonality is that they're all execution based encounters, there are no easy-win strategies. You got to learn and adjust to patterns, otherwise you're dead.
Artwise the game does a good job. Sprites and backgrounds are well drawn, animations fluid and the tranquil music fits the game's atmosphere very well. For boss fights the music gets more intense, much like we know it from the Dark Souls games. Outside of the game however I don't find the OST very memorable and worth listening to. Despite well drawn art assets the game's visuals also suffer a bit from a certain lack of color as each area has a strict color coding. For example the first area is all blue, then there's an area that's all green and some areas that are a faint grey. It looks somewhat like the assets are prepared in grey tones and shined on by spotlights of different colors for each area. It's also usually rather bleak color tones that fill the screen, but that's alright considering the setting and tone of the game.
I have to admit I'm getting tired of having to mention all the Dark Souls parallels. Unfortunately it gets really bad from here, because if you ask me the setting and lore are a bit too familiar to the well known franchise for my taste. Hollow Knight is set in the fallen kingdom of Hallownest in which almost all citizens and creatures have succumbed to an infection that practically turns them into zombies (or hollows if you will). The concepts of an abyss and kinghood also play prominent roles, and the story is told in pieces that are difficult to piece together. There seems to be a coherent lore that tells of Hallownests past and present, but good luck figuring that out. The vagueness of it all is almost as bad as in Dark Souls III. Hollow Knight is a game you would want to play for its challenging action and deep exploration aspects, the story I find barely worth mentioning.
Unlike other Metroidvania style games this one has some really difficult sections. Depending on your charm setup boss battles may leave little room for error, especially a few frustrating one's that expect you to learn their move sets very well. There's also an area late game that reminds of Super Meat Boy so good platforming skills are also expected of the player. My bottom line is that Hollow Knight is a bit too much. I came into the game expecting little challenge and maybe 8-12 hours to fully complete the game. What I got was almost 30 hours for the true ending at 96% and many very difficult boss fights. It's a very good game, but I believe it's not for everyone. Fans of Metroidvanias that don't mind a challenge will feel right at home, as will fans of Dark Souls (especially the third one). I just recommend to be prepared for a long journey and not mistake this one for a short game. Also there's been free content packs coming out long after release, thus the game kind of keeps getting bigger. I'm not a fan of this sort of thing in offline games because I don't like revisiting games when I'm done with them. Your view on this might differ.
On a side note, if you enjoyed this get Momodora IV. If you like Dark Souls and Metroid, but feel that Hollow Knight is too big of a game of its kind, then get Momodora IV as well. It's similar as it's also a Dark Souls inspired Metroidvania - but it's much shorter.