Tue 05 Mar 2024, 09:13
You may use any Fell Ability as often as you like, so long as you meet any prerequisites. Hideous Toughness is triggered every single time the prerequisites are met - potentially even 6 or more times in a round.
The only restriction beyond that is that an adversary can only spend Hate equal to its Might each combat round (p143). This is a little vague, but a common sense interpretation is that "spend" means "spend" - losing Hate due to, say, Fear of Fire, does not count. It has to be an act of agency - a spend.
So, a Hound of Sauron could choose to do two Great Leaps in a round, since it is Might 2. A more ambiguous example but one that makes sense, is that a Barrow-Wight can be affected by its Hate Sunlight (loses 1 Hate per round while in sunlight) and still use its Dreadful Spells Fell Ability. On the other hand, it then wouldn't be able to use its Deathless ability (spend Hate to cancel a Wound or to go from zero endurance to full endurance).
Personally, I think that's a little hanky, but it's the most common sense interpretation of the rules as written at the moment.