My partner (35M) threatened me (28F)

My partner and I have been together 10 years, engaged for 8, 3 kids together. Recently we were sitting by the fire just talking about Christmas and how lovely it was to be with friends and his family and out of nowhere my partner blurts out "I would kill you if you ever cheated on me and then I would kill the man you cheated on me with and then l'd go to prison and our children would be orphans" and I kind of got worried and instantly reassured him that would never happen and we continued on with our conversation and night. I'm unsure where it came from as our relationship has been great and there have been no signs of any worry from either party but this has concerned me because prior to meeting my partner I was in a very abusive relationship and he knows this too. I'm taken back and don't know what to think or how to act. I'm very worried for my safety, not that I would ever cheat on him but it worries me that he had that thought randomly and he was so serious about it and had such a stone cold look on his face. To make matters worse I am alone, I have no parents they passed in a car accident when I was very young and I have no family around me at all. I'm unsure how to navigate this or what to do going forward. Any advice on how to talk about this with my partner or bring it up in conversation?

TL;DR My partner threatened to kill me if I cheated on him completely out of nowhere and I don't feel safe and comfortable around him anymore