Spiral Knights

Spiral Knights is a free-to-play massively multiplayer action role-playing game, with a focus on instanced adventures. When the Spiral Knights crash-land on an alien world, their spaceship Skylark was badly damaged. In particular, the engine core was completely destroyed. With no rescue party expected, it is up to them to venture into the heart of the world in search of the incredible energy reading they detected from orbit. Standing in their way is the planet itself. Cradle is a rather strange planet, featuring the Clockworks, a huge subterranean dungeon that constantly moves and shifts, with more dangers on every level.

While Spiral Knights is free-to-play, most actions (traveling to a new Clockworks level, crafting, etc) require Energy. While energy naturally regenerates to maximum over the course of about a day, Crystal Energy can be bought or traded, letting players play longer, and use higher-end crafting. Traveling into the Clockworks can be done solo, or as a 2-4 player group. Each level is randomly generated, and features light puzzles and a lot of combat. While there is no player experience, each equipment item levels up from heat orbs lying around from defeated enemies.