Zeonpus was the creator of the mysterious tower that once rose above the sample starter dungeon in the Holmes Basic D&D book. All we know about him is this:
100 years ago the sorcerer Zenopus built a tower on the low hills overlooking Portown. The tower was close to the sea cliff west of the town and, appropriately, next door to the graveyard. Rumor has it that the magician made extensive cellars and tunnels underneath the tower. The town is located on the ruins of a much older city of doubtful history, land Zenopus was said to excavate in his cellars in search of ancient treasures.
Fifty years ago, on a cold wintry night, the wizard's tower was suddenly engulfed in green flame. Several of his human servants escaped the holocaust, saying their master had been destroyed by some powerful force he had unleashed in the depths of the tower. Needless to say the tower stood vacant for a while after this, but then the neighbors and the night watchmen complained that ghostly blue lights appeared in the windows at night, that ghastly screams could be heard emanating from the tower at all hours, and goblin figures could be seen dancing on the tower roof in the moonlight. Finally the authorities had a catapult rolled through the streets of the town and the tower was battered to rubble. This stopped the tauntings but the townsfolk continue to shun the ruins. The entrance to the old dungeons can be easily located as a flight of broad stone steps leading down into darkness, but the few adventurous souls who hove descended into crypts below the ruin have either reported only empty stone corridors or have failed to return at all. Other magic-users have moved into the town but the site of the old tower remains abandoned.
Whispered tales are told of fabulous treasure and unspeakable monsters in the underground passages below the hilltop, and the story tellers are always careful to point out that the reputed dungeons lie in close proximity to the foundations of the older, pre-human city, to the graveyard, and to the sea.
Portown is a small but busy city linking the caravan routes from the south to the merchant ships that dare the pirate-infested waters of the Northern Sea. Humans and non-humans from all over the globe meet here. At the Green Dragon Inn, the players of the game gather their characters for an assault on the fabulous passages beneath the ruined Wizard's tower.
So we really know very little other than that he was a wizard, he built a tower, and he was curious enough to risk his life searching for lost knowledge and power. Now in D&D a wizard is supposed to be 9th level before he builds a tower, so we have a baseline there. We also know he lived in the tower for 50 years before The Incident, so I'm going to propose that he had advanced at least a bit beyond 9th level during that time, started off youngish, and ended up oldish. The signature notes from the end are "green flame" and a force unleashed in he depths - maybe he was a summoner? So...
BECMI D&D - Let's call him a 14th level M-U with all of the fire spells and some summoning as well. All of his fire spells are customized to be green fire, not regular fire. If you can find his spellbook, you can do the same thing. Give him a 17 Intelligence and an 8 Wisdom.
AD&D - Same thing, maybe with a green efreeti servant. Engulfed in green flame sounds more like a deal with the Nine Hells or the Abyss gone wrong though so give him a quasit or imp for a familiar, a flesh golem servant, and a demon running loose in the ruins of his tower. If he's going to make a personal appearance give him a custom spell that changes cone of cold to fire damage - green fire. M-U players will love it.
AD&D 2nd edition - Make him a conjuration specialist with a lot of Monster Summoning spells and the same familiars and servants above.
D&D 3E - I'd still keep him a pure wizard with a few levels in one of the summoning or planar focused prestige classes, maybe even a few levels of Loremaster if you want to assume he has learned a few things during his excavations. I'd still go with one of the improved familiars and a flesh golem bodyguard because I think it's cool.
D&D 4th - Wizard is still good but Warlock might be more thematic, Infernal Pact or Vestige Pact warlock especially so. The green flame was the pact being called in at the end of his life on this plane in a particularly spectacular fashion.
So what ended his life? If you go with the dabbling-in-things-better-left-alone theory then I like the idea that he opened up a gate to somewhere else because he had just enough knowledge and power to find it, recognize what it was, and figure out how to open it, but not enough knowledge or power to control it. It could be the aforementioned Nine Hells or Abyss, or maybe it's the Far Realm (in 4E) or the Shadowfell / Negative Material plane which is causing the dead of the city to rise and cause trouble, or maybe he found the upper outpost of a Great Underground Empire and the thing he opened is a gate to the now-ruined capital city that lies even deeper underground and allows weird monsters to find their way into the upper levels of the underground where they would normally be seen. There are many options, some probably better for low-level heroes than others.
What if he was blown into another universe? Maybe he learned his lesson and now fights to stop those who probe into such forbidden knowledge as a mystic in the world of Mutants and Masterminds?
Set it in the Old World of Warhammer and the story works as-is and makes a pretty decent starting adventure there too.
In 40K he becomes Zenopus the Rogue Psyker, delving into forbidden knowledge and now on the run from the Inquisition, making for a fun little romp to start that campaign off.
In Deadlands he becomes the crazy lighthouse keeper who is clearly digging into things he shouldn't be and perhaps the PC's help arrange his dramatic emerald ending.
In Star Wars he could be a simple rural force-user who takes a walk down the dark side early in the campaign, appears to be reformed by the intervention of the Jedi Order, then later in the game becomes the awesome Darth Zenopus, master of force-wizardry and ranged effects and blazing green force lightning, less so the lightsaber. Perhpas after his demise some PC's are asked to journey to his lair and investigate, giving an excuse to send the heroes down some 10' wide corridors and rectangular rooms deep underground with hidden secret doors, fiendish traps, and enigmatic statues.
Anyway that's the Zenopus brainstorming session - no sheets, no serious stats, just concepts.
Tomorrow: What to do, what to do....