A form of monster species “almost grudge” - the two species will not attack each other on sight, but they are far more likely to attack each other when subject to conflict than two arbitrary monsters would. An example of such an almost-grudge would be elves and dwarves.

In variants where a monster can grab the Amulet on Astral and attempt to ascend with it: they gain the ability to shove monsters out of their way while pathfinding towards their high altar, but they are grudged by all monsters who are crossaligned with them.

Lizards and cockatrices/chickatrices should grudge each other.

Leprechauns have a one-way grudge against gold golems and gold dragons.

Monsters on the receiving end of a one-way grudge flee from monsters that grudge them.

Rust monsters have a one-way grudge against iron golems.

Giant spiders grudge killer bees.

If Astral denizens are able to steal the Amulet from you and try to ascend with it: Keep track of the god of the current amulet-bearer (whether you or someone else). Priests and angels aligned to that god grudge all others, and all others grudge them.

Vampires grudge werewolves.

Vault guards grudge leprechauns.

Unicorns of different colors grudge each other.

Monsters will grudge another monster wielding an artifact that is a bane of them.

If you attack a peaceful monster, other peaceful monsters that saw the act but grudge the first monster will not be angered.

Ideas about nerfing the player’s ability to scare off a large group of Yendorian Army troops at once:

• Non-fleeing troops grudge fleeing ones (they’re deserters).
• Troops are immune to fleeing (possibly only immune to fleeing triggered by low HP, and still capable of fleeing from other causes) if they can see a higher-ranking military monster nearby.
• It’s harder to scare troops if there are others nearby (they have effectively a higher resistance).
• Troops recover faster from being scared if there are others nearby.

This doesn’t necessarily have to be only the Yendorian Army; the rule could be extended to all monsters being “supported” by other monsters of their same species, with some special cases for things like all orcs supporting each other.

Peaceful priests and angels of your own god on Astral help you out by attacking hostiles. Possibly only if your alignment is good.

When a monster kills a monster that it grudges, it gets some bonus like a small HP boost or a level up.

Golems hate existence so much that they attack all other creatures, but the other creatures don’t hate golems, so they just defend. This is a special case of implementing one-way grudges.

Ordinary monsters should attack any player monster with the same priority as the actual player.

Tame monsters continue to hold grudges against each other, unless the master’s Charisma is high enough.