Any rules chosen must have the following criteria:
Suitable for grim sword-and-sorcery adventure. (Erebus, from the beginning, was a sword-and-sorcery place.)
Easy to learn and play. (I’m not about to push a complex system on new players.)
Well-supported by its maker and/or other companies. (The more adventures and supplements on the market, the fewer I have to write.)
Barbarians of Lemuria from Beyond Belief Games captures the feel of a low-magic, warrior-centric, swords-and-sorcery world. Its simple rules abstract out complicated skill systems of other games: 2d6 roll high, everything is a modifier, a short list of careers replaces long skill lists. It has limited support, but it’s pretty much complete in and of itself.
Crypts & Things from D101 Games modifies Swords & Wizardry to model a more sword & sorcery universe. Notably, there’s only one magic-using class (the Magician) who pays dearly to cast all but the most benign of spells. It adds a Barbarian class, removes non-human race classes, and makes the Fighter a bit more interesting.
Lamentations Of The Flame Princess is a nicely modified Old School system, from a writer and GM whose design philosophy fits recurring themes of Erebus: mystery, horror, danger, cruelty, futility, and an uncaring universe.
Chaosium’s Magic World, based on Basic Roleplaying, emulates Chaosium’s out-of-print Stormbringer/Elric! line without specific references to Moorcock. Combat tends to be lethal, adding to the grim-and-gritty atmosphere.
With only one magic system – Sorcery – other forms of magic noted in Magical Arts, notably Divine Magic, need to be revised or removed. For example, Clerics could derive their powers from sorcery, Allegiance, a port of Theism from RuneQuest (below), or something else … or they could be rare and legendary (i.e. NPC-only).
RuneQuest 6 by Design Mechanism closely resembles Basic Roleplaying, so if needed we can adapt a number of supplements for BRP and for RQ6’s immediate predecessor, Legend. Extensions to the combat system, and multiple magic systems, provide enough options for a fantasy campaign.
The choices boil down to these, in order of preference
Any of these systems would work. I’m most familiar with the BRP/Legend/RQ line, and if I wanted to eliminate PC magic entirely or impose a different system I might use it just so players have obvious “toys” (skills) to play with.
However, I think I’d have the best chance of attracting players with a D&D derivative, and of those Lot FP balances “retro-clone” sensibilities, modern improvements, simplicity, and tone. The Lot FP skill system is a nice compromise between hand-waves or attribute checks (as in Crypts & Things) and the problems with a Thief class or a full-blown skill system grafted onto classes and levels (as in d20).
If I had the time, though, I might pen a hybrid of Crypts & Things (classes and rules) and Lot FP (skills, generalized to all classes).
See the following for more detailed changes: