Neheb, the Eternal

If a creature is attacking a planeswalker, that planeswalker's controller is the defending player. If a creature is attacking a battle, that battle's protector is the defending player.

Neheb's ability checks only how much life opponents lost during the turn, not by how much their life total decreased compared to the start of the turn. For example, if an opponent lost 2 life and then gained 8 life before your postcombat main phase, you'll add {R}{R}.

Afflict causes the defending player to lose life; it's not damage or combat damage.

You get a postcombat main phase even if no creatures attacked during a turn. Neheb's last ability will trigger.

Afflict resolves before combat damage is dealt. If this loss of life brings a player to 0 life or less, that player loses the game immediately. A blocking creature with lifelink won't deal combat damage in time to save that player.

If you somehow have more than two main phases in a turn, each main phase after your first one is a postcombat main phase, and Neheb's last ability triggers at the beginning of each of them.