How does Hour of Devastation's Neheb the Eternal work with extra combat & main phases?

Question

Earlier today @Wizards_MagicEU revealed Neheb, the Eternal. This is a creature card which reads:

At the beginning of your postcombat main phase, add {R} to your mana pool for each 1 life your opponents have lost this turn.

Cards like World at War, Waves of Aggression and Relentless Assault create additional combat phases and main phases which make the turn structure look more like this (assuming you cast just one of them):

• Main phase
• Combat phase
• Main phase
• Combat phase
• Main phase

Let's assume in every Combat phase, I make my opponents lose some life.

How does Neheb, the Eternal's ability function in this structure? When does he trigger and how much mana does he add each time? I think he'd trigger in every main phase but the first and add a cumulative amount of mana each time; is that accurate?

Answer 1

Any Main Phase that follows a Combat phase is a "Postcombat Main Phase".

505.1a Only the first main phase of the turn is a precombat main phase. All other main phases are postcombat main phases. This includes the second main phase of a turn in which the combat phase has been skipped. It is also true of a turn in which an effect has caused an additional combat phase and an additional main phase to be created.

So Nehab, The Eternal will trigger for each Main Phase that is not the First Main Phase.

It's also worth noting that the mana added is based on cumulative life lost by opponents during this turn. Any life lost prior to this triggered ability's resolution adds {R} to the mana added to your mana pool.

Answer 2

Adding to Nathan Hase's answer, the life lost (and respectively, red mana added) is an absolute value at any time. What you're doing is basically checking the amount of life lost every time the abilily resolves, no matter what else happened this turn.

Let's have an example turn with Neheb, the Eternal already on the board:

• In your precombat Main Phase, you only cast Rift Bolt, dealing 3 damage to your opponent. In the Combat Phase, you attack with Neheb, the Eternal, dealing another 4 damage, and cast a Lightning Bolt for another 3 damage.
  In your post-combat Main Phase, its ability triggers, adding 10 red mana to your mana pool (because that's how much life your opponents lost this turn). You choose to spend some of that to cast Waves of Aggression (the unused mana empties from your mana pool at the end of the phase).
• Following that Main Phase, there's an additional Combat Phase, in which you again choose to attack with Neheb, the Eternal, dealing another four damage. In the Main Phase following that Combat Phase, the ability triggers to add 14 red mana to your mana pool.

For the sake of easier reviewability, let's display this example in a more compact manner:

phase          | damage dealt | mana produced
---------------|--------------|--------------
Main Phase 1   | 3            |
Combat Phase 1 | 4+3          |
Main Phase 2   |              | 10
Combat Phase 2 | 4            |
Main Phase 3   |              | 14