Could you tell me about it?
Sure. It's a twin stick shooter / bullet hell game with a Castlevania aesthetic. The twist is that instead of being twin stick, it's just "single stick" (in that you walk around and your weapons autofire).
The addictiveness comes in because enemies die and drop EXP orbs that you pick up to level up your character. When leveling up, you get a random chance to either increase the strength of an existing weapon or to choose a new weapon (or powerup). The game has a massive variety of weapons and powerups to choose from (30ish?) which makes for some really cool combos. Speaking of combos - if you have the right combination of weapons and powerups, your weapons (if at max level) can evolve into even more powerful weapons.
Enemies start out with low HP and are slow. As time goes on, the game throws more and stronger enemies at you. The goal is to survive as long as possible. You also earn money through random drops, and money is saved between games to buy powerups so that you start out stronger each time - so it has some roguelike elements as well. And there are about 20 different characters you can unlock through various means.
Boss enemies in the game also drop treasure chests in addition to EXP orbs, and each treasure chest has a chance to have 1, 3, or 5(!) random power increases for your existing weapons (and increasing amounts of gold). The animation for these looks like a slot machine, it's all very exciting.
Everyone IRL I've recommended the game to has been hooked on it. It's easily the best $3 I've spent in a very long time, and the fact that it's coming to Game Pass for free means I can recommend it to more people.