Exllusion was an online RPG first launched on September 22, 1996 (just 5 days before Meridian 59). Alongside The Realm and Meridian 59, it is one of the oldest modern (non-MUD) MMOs.
You could order the game for free, and then pay connection charges by the minute (8 yen a minute!) or a flat montly rate. It was similar to a BBS system, in that players would use a dial-up modem to connect directly to the game server. According to one website, the game supported around 100 simultaneous players. The service was discontinued on June 28, 1998.
An MMO on the PC98 - how cool is that?
Anyway, I don't think any copies of the client software exist. The server is long dead anyway, so it's impossible to play it.
Ben, I think that site you linked to is by one of the former developers. There are some really interesting design sketches.
The hdi from that site does not contain the full game, but it appears to be a map editor. You can open up a sample map and get an idea of how the game played.
Here's the hdi:
http://www.sendspace.com/file/qffdby
The procedure is a little complicated, so I'll explain.
1) Launch emulator (I used Neko), and mount system.hdi.
2) Press Enter twice to get to the command prompt.
3) Type KF, and press Enter to launch the file manager.
4) Select the MAP directory, and press Enter.
5) Select the SAMPLE.MAP file, and press Alt. A menu will appear.
6) Press Z.
This screen should appear:
Move around with the arrow keys. Tap Z and X to adjust the movement speed. If you press Enter, a status page and menu will open, but it doesn't do anything.
That's about it.