When Fleshbag Marauder enters the battlefield, it triggers.
Holding priority, activate Relic of Legends' second ability by tapping Burakos, adding ({B} magic symbol) .
Burakos triggers, giving it and all creatures you control that share a creature type with it +2/+0 and undying until end of turn.
Resolve the Fleshbag Marauder trigger from step 2, causing you to sacrifice Burakos, and each opponent to sacrifice a creature.
When Burakos dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Activate Retribution of the Ancients by paying ({B} magic symbol) and removing the +1/+1 counter from Burakos, giving Fleshbag Marauder -1/-1 until end of turn.
Fleshbag Marauder dies as a state-based action due to having 0 toughness, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter from it.
Repeat steps 2 through 6 one additional time.
Activate Retribution of the Ancients by paying ({B} magic symbol) and removing the +1/+1 counter from Burakos and Fleshbag Marauder, giving Fleshbag Marauder -2/-2 until end of turn.
Repeat from step 8.
Results
Each opponent sacrifices all creatures they control.
Infinite death triggers.
Infinite ETB.
Infinite LTB.
Infinitely powerful Orcs, Wizards, Warriors, Rogues, and Clerics except Burakos and Fleshbag Marauder until end of turn.