Losing his father so brutully :/

Arthur has already had that big trauma. He grew up without the unconditional love from his mother. During his upbringing he always felt he had to prove himself. He thought he had to earn Uther's love. This is something that has formed him and is a part of his complex personality. He is a tough guy on the outside, but not on the inside. It has made him cautios to rely too much on others. Like he felt all alone when he became the King. He had Merlin and Guinevere, but he felt he carried the burden alone. Slowly he is learning that it's not the case.
When he finds out about the circumstances surrounding his birth, it will hit him hard. As it already did once.

Morgana's betrayal hurt him but it is not big enough to be a trauma. If Gwen really had betrayed him and left him. It would have ruined him. But that isn't the case. He forgave her and married her.

I want it to be the continue of what Uther did by executing innocent people. Something like in A Herald of a New Age but when Arthur discovers that Merlin has magic, it would be even more painful and he will never forget it himself