When Melira dies, its persist ability triggers, returning it from your graveyard to the battlefield without a -1/-1 counter on it due to its static effect.
Resolve the Shilgengar ability.
Repeat.
This ingenious cycle turns your board into a self-sustaining engine, anchored by Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight granting its vital persist safety net to your team. Melira, Sylvok Outcast steps in with her crucial passive ability, completely nullifying the -1/-1 counters that would normally shut persist down and allowing Melira to return to the battlefield unscathed every single time. Meanwhile, Shilgengar, Sire of Famine fuels the entire loop by turning those repeated sacrifices into an endless stream of Blood tokens, which can eventually be cashed in to reanimate your entire graveyard.