It obviously applies a constant effect "Units in a zone with no developments receives -1 HP"

So if I recruit a 1 HP unit in to a zone with no development, is there an action window in which I can sacrifice that unit as a cost to an effect before the game state says it has 0HP and therefore damage equal to it's HP and therefore has to go to the Discard Pile?

FAQ 1.4 suggests that you resolve all Forced effects and all Constant Effects before actions can be triggered. So by my understanding not.

Also what happens if resolving an effect removes the last development in a zone while the Throne is in play?

I would expect that all units in that zone instantly get -1 HP, and where applicable are sent to the discard zone, again without an opportunity to trigger abilities that require a sacrifice to set off.

My current deck idea would REALLY benefit from being able to still sacrifice stuff (e.g. Slave Pits) but I think that it doesn't work

Gmax101 said:

You are correct on both counts. There is no time to respond if you play that 1 HP unit. It just dies. I believe that if that 1 HP unit has a triggered "when this unit enters play" effect, that effect will still be triggered as the unit comes into play, and will be resolved sometime after it leaves play.

Same for the removal of a development. As soon as the effect resolves and the development is gone, all the units in that zone now have 1 less HP. Anyone at 0 dies.

If you have Malekith's Throne Room, and your opponent has Scrap Heap, both affects are applied simultaneously and cancel each other out. This may seem like it breaks the rules of how you resolve simultaneous constant effects, but it doesn't. Those rules (FAQ v1.4 flowchart) actually only apply to *triggered* simultaneous constants. Non-triggered constants just apply all the time and impose their net effect.

A shame but makes sense

In the second instance, you would have an opportunity to sacrifice the unit as follows. Your opponent declares he's using whatever action to destroy to your development. That effect is now first in the stack. You could then choose to sacrifice your unit, adding that to the stack. As the stack resolves, you follow the last-in, first-out (LIFO) rule. You sacrifice your unit, THEN the development is destroyed, THEN any remaining units would get -1HP.

RM