This has been out for maybe a couple of years at this point, but it's recently been updated with the AG&D expansion pack, and I considered that a decent excuse to finally grab a copy.
Bit of a terrible idea, really, since I've previously been awful at these top-down, twin-stick shooters.
I still am, by the way. But luckily, this is fun.
So, the idea is that there's a dungeon, and you've got to explore it. However, since we live in modern times, there will be none of this faffing about with swords and shields. Instead, you've got the option of picking up a few hundred guns, the occasional grenade and a couple of nifty gadgets.
It's a horribly fast-paced bullet hell, with hordes of enemies of all different types attempting to murder you in dozens of ways. There's living bullets, who shoot smaller versions of themselves at you. You'll get spooked by ghosts with machine guns, slashed by guys with massive swords who somehow shoot lasers out of them, and troublesome planetary and philsophical concepts that really shouldn't be a threat but are.
On top of all of the standard monsters, there's also the bosses. The dungeon is divided into levels, and you've got to get deeper by exploring each one until you find the route down to the next. Unfortunately, before you can pop into the lift, you've got to murder a huge monster who tends to fill the screen with shiny death, in a range of difficult ways. Each has its own pattern, and will kill you a lot.
Many of these are references. Okay, a lot of these are references. It's a very nerdy game, such that it's always great fun to pick up a new weapon, and discover it's Megaman's arm or the bouncy bananas from worms. I'm sufficiently bad at the game that I've still not even beaten the second level. At some point I might unlock a lift to take me straight down to the third without having to crunch through the first, "easy", level, but I just can't kill that second boss.
And yet I keep going back to it. For a procedural rogue-like, it has a lot of additional elements that increase the complexity and make each trip very different. There's both an in-dungeon and out of dungeon shop system, to enable both in-run healing/equipping and persistent improvement of your character. There's a few different sneaky plot elements, such as Princesses locked away in cages, and you've got four characters to play as, each of which have a unique skillset.
I feel a little bad talking about this one having done as poorly as I have, but it's got me constantly going back to attack it again every time I fail.