@nik87 while i still believe Garling is Top Candidate,
Last Chapter made me Believe that the Legendary Fight ruined by some Weakling wasn't Xebec vs Garling or Xebec vs Garp/Roger,
It was Xebec vs Harald (I don't think Harald necessarily tried to Kill Xebec, but at least Prevent him from causing more Damage to WG, similar to how Noland fought Calgara),
So this means Xebec Killer/Achilles Heel (Or God's Valley Version of Mr. 3, Guernica, Flampe, Higurashi ... etc) was probably from Harald's Side,
And i can only think of Two Options:
1. It was simply Ida (Similar to Calgara's Daughter), and that's how She Died (or Maybe got Captured by WG as Slave/Prisoner)
2. There is a Traitor or WG Spy or Extremist in Elbaph who made sure Harald does his Job (After all, Mars Offer was for All Elbaph Warriors)
It could be Jarul or Road or Someone from Walrus School or Biblo The Owl ... etc