Belbe, Corrupted Observer
Legendary Creature — Phyrexian Zombie Elf
At the beginning of each postcombat main phase, the active player adds 
for each of your opponents who lost life this turn. (Damage causes loss of life.)
"Those who wish to become evincar must first cast off the shackles of compassion."
Igor Kieryluk
Commander Legends (CMR)
• #590Rulings
If an opponent lost life and subsequently lost the game, Belbe's triggered ability still counts that player to determine how much mana to add.
(wotc • 2020-11-10)
If an opponent loses life before their own postcombat main phase, Belbe will reward their suffering with mana.
(wotc • 2020-11-10)
If an effect creates an additional combat phase in a turn, it may also create an additional main phase after that combat phase. Belbe's ability triggers at the beginning of each of these postcombat main phases.
(wotc • 2020-11-10)
Rulings
If an opponent lost life and subsequently lost the game, Belbe's triggered ability still counts that player to determine how much mana to add.
(wotc • 2020-11-10)
If an opponent loses life before their own postcombat main phase, Belbe will reward their suffering with mana.
(wotc • 2020-11-10)
If an effect creates an additional combat phase in a turn, it may also create an additional main phase after that combat phase. Belbe's ability triggers at the beginning of each of these postcombat main phases.
(wotc • 2020-11-10)