Achil·les' heel 
/əˈkɪliz-/ noun
plural ∼ heels /əˈkɪliz-/ noun [count] : a fault or weakness that causes or could cause someone or something to fail
▪ I'm trying to lose weight, but ice cream is my Achilles' heel. ▪ This year, the team's Achilles' heel is its pitching. [=the team could fail because of its poor pitching]
▪ I'm trying to lose weight, but ice cream is my Achilles' heel. ▪ This year, the team's Achilles' heel is its pitching. [=the team could fail because of its poor pitching]
