When Sekki enters, Pandemonium triggers, targeting Sekki. Sekki prevents the 8 damage that would be dealt to it and instead removes eight +1/+1 counters from Sekki and creates eight 1/1 Spirit creature tokens.
Sekki dies as a state-based action due to having 0 toughness.
When the Spirits enter, Pandemonium triggers that many times, dealing a total of 8 damage divided among any number of targets.
Activate Sekki by sacrificing the eight Spirit tokens, returning Sekki from your graveyard to the battlefield.