Ashura is the fan-atoned name of the glitched-palette phenomena found in Sonic the Hedgehog 2's Emerald Hill Zone.
In order to reproduce the Ashura glitch, the zone select and debug codes must be active. Upon activation, proceed to one of the two acts of Emerald Hill Zone, as it does not occur in any other level. When the zone begins, press . Sonic will be a ring, signifying that debug mode is active. Press four times; Sonic should be an invisible object. Then, press repeatedly to create a number of waterfall objects. Press to change back into Sonic, who, if the glitch has been activated correctly, will emerge from behind the waterfalls with a green and black palette.
The reason for the Ashura palette glitch is a result of several objects being loaded into the Sprite Status Table at a given point in time. Since RAM on the Mega Drive is in limited quantity, only so many objects can be loaded into the status table before it reaches its allotted limit. When a mass of waterfall objects are placed using debug mode, this overloads the SST to a point that exceeds the SST's reserved RAM area; since the palette cache used by the game to update CRAM is located after the SST in system memory, SST data becomes parsed as color data in the reserved palette cache area and is loaded into CRAM accordingly. Because of this, the green color is purely a coincidence--it is not intentional.
Similar palette errors can occur upon porting the Sonic 1 sound driver to Sonic 2; if Sonic loses rings whilst underwater, it will overload the SST and spill into the palette cache, corrupting the underwater character palette line.
Kushami, a member of the Sonic Retro forums and a veteran of the online Sonic scene, posted the following first person explanation of the origins of the Ashura glitch:
|| This is the sordid, unimpressive tale of Ashura:
Charles "Kiken" Mugg used to run a Sonic website on his college account in the early 90's which was devoted to the Japanese stuff from the games. This is something which was pretty rare back then; Most other sites of the time focused on the Satam Show and Archie Comics - such as rat.org/sonic - if anyone here is old enough to even remember that. He had a bunch of scans from fliers and stuff, and he did a bunch of translations from the Japanese Sonic CD manual. Out of the 5 or 6 Sonic sites which existed back then, his was the only site which had content that was, I guess I'll use this term "SegaSonic."
Later on he discovered the pallet glitch, and decided to post the trick on his website and named it "Ashura." He also posted a lot of other stuff you could do in the games along with it; It's where a lot of the first debug tricks showed up on the internet, such as the stuff in Sonic 3's Angel Island zone with Sonic on a surfboard, the weird 3D balls, and on and on. I submitted some of that stuff, ironically. This is also where the first scan of the SegaSonic arcade game showed up, along with the character names and a little other info. This stuff is all common knowledge now that the games have been completely emulated, ripped apart, and disassembled, but this was something like 1993 or 94 at the time, and most computers struggled with even emulating Genesis games.
Around the time he posted the glitch, I found his page and became friends with him. I thought the glitch was cool (I guess I was 12 or 13?), and I asked him if I could use the name as my handle on the internet. To my surprise, he said sure thing, go for it. I've used it as a handle for the most part since then, and he's been my good buddy for something like 13 years. (I'm math challenged, so that could be wrong.) He also drew the artwork of "Ashura" with the bang that floats around in ultra bad 1990s scan quality. I have the original of it, somewhere... and that's pretty much it.
He (and I) had nothing to do with the crazy amount of weird bullshit that's appeared after the above events. People just ran with it, I guess? It amazes (and horrifies) me to see what people come up with when I google it. Sometimes it's like my eyes are being raped by the awful powers of awfulness. I find the fact that people feel so strongly about the whole thing either way is pretty hilarious, though.
—Kushami, Sonic Retro Forums
Though only a palette glitch, Ashura is one of the four characters possible in Yuski the Dog's SMS ROM hack, Sonic vs. Os Mauniks. Ashura was even hacked into Sonic R  and Sonic Heroes . Also, there is a way to get an "Ashura"-esque Chao in Sonic Adventure 2: Battle; it is the dark/run Shadow the Hedgehog Chao. Additionally, Ashura is the titular co-villain in the fan-made game Ashura Dark Reign.