I have been having a lot of fun lately creating a lot of custom heroes (more than 20), but since I am the playing as the Overlord every time, I have been thinking about making abilities for monsters. The abilities below are going to be put in use in custom quests and given to homemade monsters made by myself (I will make another thread for those, when I stop being lazy and actually make custom monster cards) or hell, even by you. Anyhow, here they are.
Flavour text for each ability is in italics, then concrete explanation is written in bold. Finally, my comments are written in normal font.

• Ravage: Some creatures possess deadly claws or fangs, or fight in a way that manages to bypass someone's defenses altogether.

— Effect: After calculating damage caused by a monster with Ravage , add 1 "true" damage. "True" damage may not be blocked, absorbed or cancelled in any way.
—> For each rank of Ravage above the first, add 1 true damage.

Beastmen are cool monsters but. I feel they suck big time and pose no threat after mid-game. The idea I have with the Ravage ability is to have low-tier (monsters such as Beastmen) not ignored by the heroes, as Ravage may inflict some damage.
I had in mind to create Ghoul monsters. Ghouls would have stats more or less equivalent to Beastmen, but with Ravage 1 (for regular Ghouls) and Ravage 2 (for master Ghouls) instead of Pierce.

• Moult: Reptilian and insect creatures are able to abandon their skin or external husk for a new one through moulting. Thankfully for them, nature made them able to entirely ignore and forget past wounds and almost start over a new life.

— Effect: A monster capable of Moulting may recover all of their health at the beginning of the Overlord's turn as long as they have not attacked or taken damage for a round (a round is the Overlord's turn + the heroes' turn).
—> No additional ranks of Moult may exist.

Wound monsters gifted with Moulting should be the main target for the heroes as they may come back to full wounds if they are ignored after taking some damage.

• Corruption: The darkest magic is able to slowly corrupt its victim, painfully tormenting him or her into a long-lasting pain.

— Effect: When a monster with Corruption inflicts at least 1 damage to a hero, a Corruption marker is placed on this hero. At the beginning of the heroes' turn every afflicted hero removes one Corruption marker. Each Corruption marker removed in this fashion inflicts 1 wound to the hero (ignoring armour).
—> For each rank of Corruption above the first, target hero receives 1 additional Corruption marker when Corruption takes effect.

I for one do not enjoy "luck" rolls a lot. What I had in mind when creating Corruption was to have a damage-over-time ability that kind of works like Burn , but taking away the luck factor. With Corruption , what you see is what you get. Also, the ability happens to have a lot of synergy with...

• Painful Expulsion: Instead of watching them slowly die, some monsters would rather see them cry in pain right away. Such pain is so unbearable that the victim is forced to scream, also afflicting his or her nearby friends.

— Effect: When a monster with Painful Expulsion inflicts at least 1 damage to a hero having Corruption markers placed upon him, every Corruption marker is immediately removed from the hero. For each marker removed in this fashion, the hero takes 1 damage per removed marker (ignoring armour). Also, any hero that happens to be adjacent to the hero victim of Painful Expulsion also takes damage based upon the number of Corruption token removed from the first hero (on a one-for-one basis).
—> Each additional rank of Painful Explosion increases the effect's range by 1.
If a monster happens to have at once Corruption and Painful Expulsion , the Overlord may choose for each of that monster's attacks which of the two abilities is used.

I think it'd be fun to have monsters with Corruption and monsters with Painful Expulsion together on the same map for some fun mayhem. Monsters having both of those skills should be bosses or weak enemies.

More custom monsters abilities to come soon

• Ravage: Some creatures possess deadly claws or fangs, or fight in a way that manages to bypass someone's defenses altogether.

Beastmen are actually THE best monsters in the game, they don't need to be any better. To see this on a different creature though... hummm doesn't seem any more unbalanced or threatening than creatures with already insane pierce numbers or bleed. Would go around certain specific circumstances like potions and feats though.

• Moult: Reptilian and insect creatures are able to abandon their skin or external husk for a new one through moulting. Thankfully for them, nature made them able to entirely ignore and forget past wounds and almost start over a new life.

This won't work. Monsters in Descent die in one round, alternatively in two rounds if they are lucky. The only case when this would do anything at all would be with boss monsters who cannot be killed in one round but that would effectively lock the heroes in a situation where they can't win. In short: useless for regular monsters, game breaking for bosses.

• Corruption: The darkest magic is able to slowly corrupt its victim, painfully tormenting him or her into a long-lasting pain.

There is already an effect called Corruption in the game.

• Painful Expulsion: Instead of watching them slowly die, some monsters would rather see them cry in pain right away. Such pain is so unbearable that the victim is forced to scream, also afflicting his or her nearby friends.

This actually sounds a interesting.

ravage: i don't see a big difference to pierce. you're implying that in the late game beastman don't even do a single damage, but the heroes armor doesn't increase that much (if at all) during the course of a game. adding pierce 1 usually makes no difference to adding 1 "real" damage, except for very weak monsters (skeletons for example, but these can just target weaker heroes).

concerning this, if you have road to legend (or downloaded a monster stats table) i can offer you our houserule:
"all monsters in the boss room have silver level stats." this can be tweaked to match opened chests etc. note that this rule makes it significantly harder for the heroes.

moult: this is the same as regeneration (existing ability, i think in RtL) with rank equal to the monsters toughness.

corruption (besides the needed name change): as long as you don't get more than 1 corruption marker per round, there's no real difference to immediately adding 1 more damage. i think this might be nice, but only if the monsters added more than 1 marker per attack (at least 3) to get a "continous damage" effect.

painful expulsion: i think this would work better without the requirement (which makes this ability too situational):
"When a monster with Painful Expulsion inflicts at least 1 damage to a hero, any hero that happens to be adjacent to the hero victim of Painful Expulsion also takes 1 damage per rank of Painful Expulsion. this damage ignores armor." sounds like a nice new area damage effect that works without any template!

A couple vanilla beastmen with a master's command ability is pretty nasty.

Frankly, these ideas for corruption and ravage would not be welcome in my game, as they're basically an attempt to create rockproof scissors. Some characters are chosen specifically based on their ability to shrug off damage, and these abilities just let the overlord peck those heroes apart.

As an OL, my biggest problem isn't that I can't get through the heroes' toughness, but rather that the heroes can one-hit so many of the monsters.