Adventurer Conqueror King AKA ACKS by Autarch is another those old school games that I’ve had for a long time but never really played as much as I wanted to.

At it’s core ACKS is another of the old school games that doesn’t try emulate a specific edition or version of D&D. It takes bits and pieces and sows them together into a unified system. There’s a total of 12 classes in the core and race is class but there are different classes for your common races. And there’s a good variety of classes too.
There’s on optional Proficiency system that is sort of a cross between a Skills and Feats. There’s the usual things you would expect for skills but there’s also other Proficiencies that act more like Feats such as Black Lore of Zahar (necromany type stuff), Combat Trickery, Divine Health, and Swashbuckler. While I find it inspirational, I personally think it’s a bit cumbersome. I’d much rather have the two split into two different things.
ACKS has the usual array of spells, monsters, and campaign and dungeon construction advice. ACKS goes into much more detail into so-called domain level play than most other games. You know your character now has a tower or is leading an army, a thieves’ guild or something like that. This is probably why I never played it that much. I’m just not into domain level play. I’d much rather be crawling through dungeons than messing around with accounting and stuff. That doesn’t mean that it’s wrong, it’s just not my cup of tea. That being said, ACKS does have a strong built-in system for domain play with extensive price lists, tables and charts. So if you like domain play, it’s worth it to pick this up for that resource which you could easily use no matter what other game system you may be actually playing.
So how do I use ACKS? Well, as I mentioned the Proficiencies are also sort of like Feats. It’s great inspiration on how to build or add special little abilities for characters. There’s also out of the norm classes like the Blade Dancer which can easily be converted over to your game of choice.

And of course there’s plenty of adventures and sources books. My personal favorite is Barbarian Conquerors of Kanahu since I more into sword and sorcery than high fantasy. And bonus Barbarian Conquerors has got a little gonzo built into it with alien technology.
Like very other game that I’ve ranted about, YMMV. But there’s plenty of material there to be inspirational and convertible to your rules of choice. You can check out their products on DrivethruRPG and their own site.