The mod, which was provided to livestreamer BaerTaffy by a developer who goes by joey4track, consists of a full audiovisual reskin of Spelunky. The game's brown caves become purple, while enemies and other parts of the world appear as Metroid analogs. The developer also brought in the series' music — the first level features the title theme from the original Metroid on NES.
Joey4track has not yet made the full mod available publicly; he said he probably won't release it until he's done reskinning all four of Spelunky's main areas. For now, you can check out his thread in the Mossmouth forums and watch a half-hour playthrough above.