It's a top-down twin-stick shooter gauntlet consisting of 8 waves of 100 enemies each, each wave getting progressively more difficult. You play as a sword-throwing hero trying to survive all 8 waves. There are several enemy types and each has its own ability. Skeletons throw swords, ghosts can teleport closer to the hero, crabs take more hits and can sometimes become faster, and bombs explode when struck with a sword.
The arena is infinite. The hero can run, but they can't run forever because the enemies will enter a catch-up state if they're too far away (that means they move too fast to outrun until they're back on-screen), and additionally, enemies spawn in the direction the hero is headed. Tip: don't keep moving in one direction for too long.
As you hit enemies with swords, you'll notice a meter fill up. Once it fills up, your hero gains +1 piercing, with a maximum of 8 piercing. What is piercing? This is represented by the sword icons and it's how many enemies the hero's swords can hit before disappearing. Piercing is more effective against groups of enemies, so the best strategy, especially in later waves where the enemies have more HP, is to clump them all together.
(note: try to keep the enemies on-screen, swords disappear if they're outside of the viewport)