Merciless Executioner 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
Merciless Executioner 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
Merciless Executioner -1/-1 until end of turn.
Merciless Executioner 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
Merciless Executioner, giving
Merciless Executioner -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