From the publisher's website*
We've have created a free add-on module, The Antagonizer, for experienced Ascendancy players who want a greater level of hostility and difficulty. You can download it from our Web page.
*via The Internet Archive WayBackMachine
User Summary
Your rival species get decidedly more prickly when the game is run from this executable, making diplomacy tougher and armed conflict more likely.
The expansion is commonly referred to as an "AI patch," which is something of a misnomer. It addresses no bug in the program**; it's just that fans tended to feel the game was too easy. The Antagonizer module makes the competing alien races generally disagreeable by default, making it harder to establish, and then maintain, peaceful relations. It provides an alternate play mode and does not essentially function as a patch, hence its adoption of a unique title name.
**In fact, it was bundled with a pre-existing, unrelated patch. This may have been the source of some confusion over the issue, and disagreement on the point persists. Nevertheless, the publisher's own download page unequivocally described The Antagonizer as an "add-on module."