Hollow Knight is a 2D, Metroidvania action-adventure game, which takes place in Hallownest, a fictional ancient kingdom. The player controls an insect-like, silent and nameless knight, to explore the underground world. At the beginning of the game, the knight can walk left and right and jump, similarly to many side-scrolling video games. The knight wields a nail, which is a cone-shaped sword that the player can use in combat and to interact with the environment, for example, to reveal secret areas.
In most areas of the game the player encounters hostile bugs and other sorts of creatures. Combat in the game involves using the nail to attack enemies from a short distance, as well as spells, allowing for long-range attacks. Defeated enemies drop in-game currency called "Geo". Geo is represented as small stones, made of fossilized shells. By striking enemies with the nail, the knight gains Soul which is stored in a Soul Vessel at the top-left corner of the screen, of which only a limited amount can be carried and it is consumed to use Spells and a self-healing move called "Focus". During the game, the player unlocks several offensive spells, but initially the knight can only use Soul to "Focus" and regenerate mask shards. The knight has a limited number of mask shards, which represent hit points (HP) of the character. When the knight is hit or takes damage from the environment, for example, by falling on spikes or getting struck by an enemy, a mask is destroyed. The player can regenerate masks by using "Focus". If all masks are lost, the knight dies. The game continues from a last visited bench, which work like checkpoints, however the player loses all Geo held at the moment of death and can carry a Cracked Soul Vessel which can hold 2/3 the amount of Soul a full Soul Vessel can hold. A Shade will appear at the place of death, which the player will need to defeat in order to recover the lost Geo, as well as the full ability to carry Soul.
Many areas feature more difficult enemies, referred to as bosses. The player may need to defeat them in order to progress further or to acquire new abilities. However, fighting some of them is optional. Later in the game, the player acquires a dream nail. It enables the player to face more challenging versions of some bosses which are called "Dream bosses" and they go through slight changes in appearance and names.
The player encounters non-player characters (NPCs), with whom they can interact. These characters may talk to the knight, providing information about the plot of the game or its setting, or offering aid. The player can spend the acquired currency Geo to buy items or unlock some abilities. The player can upgrade the knight's nail to deal more damage, gather more mask shards to increase the number of hits they can take or find soul vessels to carry more Soul. During the course of the game, the player acquires items that provide new movement abilities, such as the Monarch Wings, which allow the player to double jump, the Mantis Claw, which provides a wall jump, and the Mothwing Cloak that enables a quick dash; other combat abilities, known as nail arts, and the aforementioned spells are also available.
To further customise the knight, the player can equip various Charms, which can be found or bought. Charms vary in their effects. Some of them include: improved combat abilities, more masks or their regeneration, better movement skills, more powerful spells, easier collecting of currency, transformation into other forms. Equipping Charms takes up slots, called notches, limiting the number of Charm the knight can carry at the same time. It is possible to equip more Charms than notches available by "overcharming", which causes the knight to take twice as much damage and gives the health bar a purple aura.
Hallownest consists of several large, inter-connected areas with unique themes. Although Hollow Knight does not bind the player to one path through the game, there are obstacles that limit the player's access to an area. The player may need to progress in the story of the game, or acquire a specific movement ability, a magic skill or an item to progress further. To fast travel through the game's world, the player can utilise Stag Stations, terminals of a network of tunnels; the player can only travel to previously visited and unlocked stations. The player is not required to explore the whole world of Hallownest to finish the game, but it is necessary to reach the other two endings.
As the player enters a new area, they do not have access to the map of the surroundings. The player needs to find Cornifier, the Cartographer, in order to buy one. This includes following scattered papers and listening for Cornifier's humming. In order to make the map more accurate as the player explores the area and to see points of interest, the player will need to buy specific items (Pins) from the shop of Iselda, Cornifier's wife. In an update "Map Markers" are also sold by Iselda which is used to mark points on the map. The knight's position on the map can only be seen if the player is carrying a specific Charm.
Along the way, the player encounters benches. The knight can rest on them, regenerating the knight's masks, updating the map, and saving the game. In case the knight dies, the game resumes from the last bench on which the knight sat.
Hollow Knight received "universal acclaim" reviews from critics according to Metacritic with a current Metascore of 90/100.