The Atlantean RPG was first published in 1983 by Bard Games as the "Compleat Series"...The Compleat Alchemist, The Compleat Spell Caster, and The Compleat Adventurer. It was later expanded into the "Atlantean Trilogy" consisting of The Arcanum (the main rules, including all character classes), The Beastiary (the monster book), and The Lexicon (the oddly named setting book, giving a decent overview of the whole game world, although not much detail on any given area).
I first ran into the Arcanum around 1987. A friend of mine had it (as well as a ton of other good stuff...I was really envious of his gaming library at the time), and while I was fairly prejudiced early in my gaming career against any game that wasn't D&D, and particularly those games that were very similar to D&D, I became intrigued by some of the changes the Arcanum had made. Eventually, we made characters and ran at least one Arcanum campaign. I know I made three different characters, but I don't remember if it was because one died, or because I got tired of one character, or if the group as a whole got tired of the campaign and started a different one later. We never used the Lexicon, and I don't recall if the DM used the monsters from the Beastiary, the AD&D Monster Manual, or some combination of the two, but the Arcanum had made enough of an impression on me that I kept my eyes out for all three books for years, and finally bought them from Noble Knight Games a few years back. For what it's worth, at least one of those Arcanum campaigns we played was set in the Forgotten Realms, which was also my first exposure to that idea; the Forgotten Realms never made the same kind of impression on me as the Arcanum did.
So what makes this game different from D&D? I'll save details on the races, classes, and magic system for later, but here are some quick bullet points about the game:
- Point-buy stats: At a time when AD&D classes had stat requirements that could be frustratingly difficult to attain randomly, you were assured of getting the class you wanted in the Arcanum. There was some minor randomization involving the number of points you had to spend, and if you wanted a stat at the racial maximum, you only had a 10% chance to get it (otherwise you had one point less), but there was no chance of rolling straight 8s and having to play a dumpy character while your friend rolls a bunch of 16s.
- Speed and Perception stats: The Atlantean system had eight stats instead of D&D's six. Most were the same, with Will and Wisdom as obvious analogues, but the Arcanum added Perception (allowing characters different chances to notice an ambush or find a secret door...yes, Elves had a good Perception maximum) and Speed (giving characters different base movement rates).
- No combat tables: By the time I ran into the Arcanum, THAC0 was a well-known concept in D&D. THAC0 requires subtraction,though, and while that's not overly hard, it is still harder than adding. In the Arcanum, 11 is the number to hit, and characters get periodic bonuses to hit based primarily on class and level, similar to 3rd Edition BAB. Also saving throws don't have a table, but are based again on the number 11, with bonuses or penalties based on stats. If it makes sense to dodge a trap, save vs Dexterity. If it makes sense to tough it out, save vs Constitution. There are really no save-or-die effects, so while higher level characters fail their saves more often than in D&D, it doesn't matter as much. Also, lower level characters are more durable because of this.
- Characters are more durable at lower level: Besides easier saving throws and fewer save-or-die effects, characters have more hit points. Hit points for all characters are equal to their Constitution score, with an additional fixed amount per level based on class, plus a bonus per level for high Constitution. Even a pretty scrawny mage-type ends up with at least 10 or 12 hp at 1st level most of the time, and since it's a point-buy system, if you feel screwed over, it's your own fault. No worries about getting mauled by the innkeeper's cat and left for dead.
- Armor absorbs damage: While I've mellowed towards the abstract nature of D&D armor classes, at the time, I was unhappy with it, to say the least. I suppose it comes from the idea that you need to roll over a certain number to hit, and therefore if you roll too low, it's a miss. This in turn leads to the idea that there are to hit rolls that will hit a nimble, dodgy thief with a 5 AC, but miss a guy loaded down in platemail and 80 pounds of gear. The Arcanum foreshadows later editions of D&D in that heavier armor penalizes Dexterity rolls.
- Characters are more customizable: Most of a class's description breaks down to a list of skills. These skills can be bought separately by spending experience points, so if your paladin really needs to know how to sneak around, he can delay gaining a level and learn Stealth. Spellcasters can even learn spells from other lists.
- No more single-use magic users: All spellcasters have (level+2) spells per day, so even 1st level characters can cast three times before they're out. Even at that, the skills associated with each class give them something more they can contribute, usually including some sort of minor alchemical ability.
- "I don't want to be the cleric!": Parties without a cleric in D&D spend a lot of time holed up licking their wounds. It's hard for a small group of heroes to keep pressure on a large group of bad guys if they have to take a few days between fights, so someone has to be the cleric. Not everyone wants to be a cleric though, and sometimes it gets passed along as a rotating duty from campaign to campaign. In the Arcanum, any character with Divine magic, Elemental magic, Low magic, Healing Arts, or Herbal Remedies can heal at 1st level, so whoever's turn it is to be the healer has a choice of 10 different classes. More characters pick up healing abilities at higher levels, too.
- No walking magic-item hoards: All my early games of D&D and AD&D seemed to end up swimming in magic items. Most of them ended up being kept around "just in case", charges hoarded away until the absolute perfect time, which kind of just added to the problem. (Encumbrance should have kept this under control, I suppose, but like many people, we usually ended up handwaving that in the interest of getting on with the game.) The Arcanum specifically forbids any character from carrying more than seven magic items or they ALL stop working (one use items like scrolls and alchemical mixtures excluded). Still plenty of room for a magic shield, magic armor, magic boots, magic sword, dagger, spear, and magic helmet. But when you find a ring or something, you need to prioritize what you want and what you can do without...