Michael Moon will be reunited with Eddie next week when the ex-boxer arrives on Albert Square unexpectedly, keen to catch up with his nephew Alfie and meet baby Tommy for the first time.
Eddie (David Essex) is unaware that Michael is also in Walford and as the pair come face-to-face again, viewers will see that father and son are not on speaking terms.
Shepherd told Soaplife of the reason for the mysterious grudge: "It's something dark and it's still very raw for Michael. It's made him the person he is today. It's why he is so damaged. And dangerous."
The actor added that the Moon family reunion could have a "damaging effect on Michael's relationships with the people around him" as Eddie's return has stirred up bad memories.
Asked whether Michael lets girlfriend Roxy know why he hates his father, Shepherd replied: "At first, no, but he does eventually and she shoulders some of the burden. But that won't be enough for Michael. He'll want to exact some sort of revenge on his dad. It will have ramifications for everybody involved."
EastEnders bosses are currently keeping the reasons behind the family feud under wraps.
Read more Eddie Moon coverage:-
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> EastEnders reveals new Moon arrivals
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> EastEnders interview with David Essex
Watch a preview of Eddie's arrival in Walford in the video below: