Overview

Sonic Eraser was a video game that was downloadable for players that had the Sega Meganet, a modem for the Sega Mega Drive in Japan. It was released in 1991. While technically is a title in the Sonic the Hedgehog series, his inclusion on the game looks more of an afterthought, since other than his sprite, there is little that relates the game with the rest of the franchise.
Since the Meganet modem never achieved widespread noterity, Sonic Eraser became a "long-lost" Sonic game, i.e. the most unnoticed Sonic game ever among English-speaking audiences until February 2004, when Eric Robert Gray (also known as Pachuka and Reala), the owner of the Sonic Cult website, got a hold of the ROM through SEGA's Japan-only download service.
The game is a fairly simple puzzle game. In the versus mode of this game, when a player gets a combination of three consecutive lineups of pieces, that player's Sonic attacks the other player's Sonic. The other player momentairly loses control of his pieces.
The game is on Sega's B-Club download service.

Gameplay Modes

The game offers 4 unique modes of gameplay.

Round Mode: This is sort of a puzzle mode consisting of 10 stages. Each round has a bunch of whirlybob ring shapes interspersed with other normal shapes. Just like the other shapes, the whirlybobs disappear when paired. The object is to clear them all from the playfield by eliminating the shapes between them. The time limit is 3 minutes for each round, and keep in mind that pausing the game doesn't stop the clock, so you'd better remember to go before you start. If you've blown the round (i.e. if you're stuck with only 1 whirlybob left), press A and C simultaneously to end the round.

Normal Mode: This is just the standard low-pressure game. Play for points as long as you like. The better you perform, the higher your level will rise. As you climb the ladder, the speed at which clusters fall increases.

Doubt Mode: The rules are normal except that white squares will never fall. Instead, a single shape from almost every cluster will turn into a white square when it hits the ground.

Block Mode: This mode defies all Newtonian physics by having clusters stand on-end. Stack 'em up as high as you dare for massive chain reactions. It only takes a single pair to send the whole tower tumbling.