This UberASM will create an effect similar to what can be seen in SMB3's Dark World, where the map will be pitch black with a small circle of light following Mario. Additionally, there's an optional feature where the light will increase in size when certain events are activated, and the effect will go away completely when another event is activated, inspired by Pokémon Emerald's Battle Pyramid gimmick.
By default it won't affect layer 3, so the border stays visible, but it can be changed. For all the possible settings look in the asm file.
Since it uses window 2, it works nicely with the original Overworld HDMA effects (save/2P prompt, submap switching transition), but it includes a hijack and some hex edits (applied automatically) to prevent those from disabling the effect. If you remove this UberASM from your ROM, patch the included "unpatch.asm" file with Asar to undo these edits.
By the way, the "increase light" effect happens as soon as the OW fades in: in the GIFs it happens after the event because of the Event Debug patch.
Note 1: if the circle is too large, it can cause the game to lag when it needs to be updated (not on SA-1), so keep that in mind. Additionally, very large circles may have some "glitchy" looking results at the very edge of the circle, which may be noticeable for a split second when the "make the effect go away" event triggers if Mario happens to be towards the top of the screen.
Note 2: if used on the main map, the circle will stay fixed in place during the "look around the map" state. Either disable it, or keep it to allow the player to look at what's ahead from a small view.
Note 3: if set to cover layer 3, it will also cover stuff like the save/2P prompt letters.
Note 4: it uses an HDMA channel, by default 6, and 453 bytes of free RAM (mostly for the windowing table).
Surprised this wasn't a thing on the site already, but it's definitely welcoming. Beyond the mentioned bugs in the submission, the only additional ones worth mentioning are that using a star warp will making the HDMA all cut up near the top of the screen (akin to note 1) and that a walking transition between two dark world overworlds with the layer 3 hidden will still display it briefly during this transition. Still accepting since the former is similar to an already mentioned bug and the ladder only occurs in that specific situation.