Fairy Tail: Final Series Episode 13 / S9 EP 290 English subbed
“Fearī Hāto” (妖精の心臓)
In X697, Zeref returns Mavis’s body to Fairy Tail’s acting guild master, Precht Gaebolg. Sensing that Mavis is in a state between life and death.
Precht puts her in stasis beneath the Guildhall and attempts to revive her, eventually discovering her curse and falsifying her death to keep it a secret from the guild.
Thirty years later, Precht’s continued revival efforts produce Fairy Heart, a source of limitless magic power.
Back in the present, Mavis surmises that Zeref intends to use Fairy Heart to defeat Acnologia, making himself an unstoppable force.
Natsu reveals he has obtained a secret technique from his year-long training that he believes will defeat Zeref, who simultaneously assembles the Spriggan 12 for war.
Meanwhile, Acnologia decides to participate in the oncoming battle.