Undercrawl is a roguelike dungeon crawler from indie developer Monster Shop Games. It's a genre that's popular in gaming in general but features even more frequently on mobile devices since the shorter, 'run' style of playthrough suits playing in small bursts that phones are often used for.
It's a very straightforward game in concept then. You'll make your way through a series of randomly generated dungeons and battle against the various creatures that reside there. The aim? To level up and emerge with enough gold to buy some upgrades before diving back in and doing it all again. The game is endless so you can keep on plundering until you're eventually defeated.