HP thresholds vs per-round actions

I was looking at the Zombie Clot adventure (really, encounter), and it has a feature I was pondering: at initiative 20, it spawns minions. This has a snowballing effect, because you get a number of waves of minions equal to the number of rounds it takes to beat the boss. This means that if the fight was already hard, minions make it much harder, but if the fight was easy, minions barely affect the difficulty.

There's an alternative method, commonly used in video games: effects (such as minion spawning) occur at hit point thresholds, rather than based on timers. This generally makes difficulty more consistent and makes "focus fire the boss" less obviously optimal tactics. This can also be used for other abilities (for example, rather than giving a dragon's breath recharge 5-6, give it recharge after a short rest or when first bloodied). On the minus side, for a lot of creatures this is quite gamist.

So, how do other people feel about this kind of threshold mechanic?

Well, focus fire on the boss is an effective strategy for a number of reasons. Summoning minions at HP thresholds won't deter anyone from that. I don't think anyone has the mentality of "Oh man, damaging the boss summons minions, so I guess I just attack something else" The boss is a dangerous creature usually, and defeating it is a priority.

It is an interesting idea though. I might try that out. I do think that limiting the creature's signature ability to thresholds is a double edged sword. It makes it more cinematic, with heavy blows being retaliated against, but it can also limit the monster by a lot. Giving a dragon 1-2 uses of it's breath per use can destroy it in some encounters, like a skirmishing dragon running hit and runs against defenses or in the wilderness. I think maybe keeping the 5-6 but also adding the bloodied thing would be a good indicator to the players that they're making progress, but I think that if you really wanted to emphasize that, you should add more than just an automatic breath recharge. Perhaps also an increase to speed, armor class, and/or a new ability would emphasize that kind of threshold combat a little more. 

My first thought is that having a Boss use some legendary ability at 75% of max HP, 50% of max HP, and 25% of max HP, with each ability becoming more and more dangerous, seems like a reasonably efficient way to do it.  The recharge roll, like for a dragon's breath weapon, I wouldn't use for a Boss fight. Because what if the Boss uses their big signature ability once, and then the dice just never let them do it again? The players might totally floorwipe the Boss and you'll be like, "If only he had gotten his recharge roll!"  

On the other hand, there's already a lot to keep track of when running combat, and there's already a lot of math happening. Trying to keep track of HP percentages might be a bit much. I'd be afraid of ending a turn and then realizing, "Oh crap! I was supposed to do a thing that round!"  And the players never really know how many hit points the Boss has. The only hit point that really matters is the final one.  So I'd prepare three special legendary abilities, each stronger than the last, that the Boss would unleash as reactions at approximately 75%, 50%, and 25% health.  

But there's one more thing. During combat, the players will have moments of triumph, like scoring a critical hit, and moments of despair, like when a big spell fails or something. I wouldn't use a legendary ability as a reaction to a moment of triumph, because that would feel like I'm trying to rob the player of their triumph. Instead, I would try to use the legendary abilities as reactions in those little moments of despair, to really ratchet up the stress and to drive home the dangerousness of the Boss. 

Either way, yeah, periodic special abilities will make the Boss feel unique, and will keep the players on their toes and stressed out. So whatever method works for you, I hope your players enjoy the challenge.