The Arduin Trilogy (also called the Arduin Grimoire) is a supplement and set of rules variants for OD&D, and can be used with pretty much any OSR game, transforming that game into the Arduin game.In a sense it's the original fantasy heartbreaker, but is also a lot more than that. The first 3 books are d20; books 4-9 move the rules gradually into a %-based system.
The Compleat Arduin is Mark Schynert's re-edit and compilation of all the Arduin Trilogy / Grimoire material, and forms a complete standalone game. It's mechanically quite detailed and complex, and very crunchy, but it's still very much Arduin -- all the essential elements (classes, races, monsters, spells, criticals, items, etc) are there. It's also much more complete than the Trilogy. It's a nice set, but it feels very much a product of the early 90s, in the same way the Trilogy feels like a product of the later 70s.
They're both cool and worth having imho. :)