The most overtly horrifying reaction DD ever had was a biphasic phase II at 2yo.
time to onset was 4 hr on the dot from the initial reaction, which was so non-specific as to not be fully recognizable as "allergic" in nature.
The late phase was unbelievably fast (seconds to full anaphylaxis) and I still have trouble wrapping my head around how severe that reaction was-- it was hard to tell whether DD was choking on mucous or if her airways were that swollen... but she was COVERED with hives-on-top-of-hives, vomiting, and then... more ominously, barely conscious.
I've never-- ever-- been so glad to live 90s from the emergency room doors. NEVER. Longest car ride of my life.
We've also seen biphasics from allergy shots-- those tended to be 8hr on the nose. It was so predictable that for a while, our allergist had us on his "call in the morning" list the day after an allergy injection. (I
him so much.)