Temple of Atropos

Even if you don't attack with any creatures during combat, you'll still get a postcombat main phase and Temple of Atropos's ability will still trigger.

If multiple phases are added to the same point in your turn, the most recently created phase happens first. For example, say the current plane is Temple of Atropos and its ability triggers during your postcombat main phase. Later during that same main phase, another effect gives you an additional combat phase after this main phase. The additional combat will happen first, followed by the additional beginning phase.

The additional beginning phase will be a lot like your normal beginning phase. During the untap step, permanents will phase in or out as appropriate and you'll untap your tapped permanents. Anything that triggers "at the beginning of your upkeep" or similar will trigger during the upkeep step, and you'll eventually draw a card for your draw step.

If the turn order is reversed during an extra turn, the next non-extra turn will be taken by the next player in turn order from the last player who took a non-extra turn. The order of any pending extra turns isn't affected.

Reversing the turn order has no effect on the order of steps and phases in each turn. Those steps and phases still happen normally (along with the additional beginning phase).

The additional beginning phase all happens during the current turn. Any effects that last "until your next turn" or similar won't expire just because you'll go through an additional beginning phase.