Probably Henry VIII's personal quillon dagger from the early 16thC.
This dagger is very closely based on a piece held in the Royal Armouries in Leeds and was once owned by Henry VIII and dated early 16thC. Probably German and features a chiseled steel pommel and guard with a delicate shell shaped nagel. Strong single edged hollow ground blade.
Grip is ebony and sheath is dark blue double layer veg tan leather with bronze chape. (Cap is speculative as original is missing, but is in line with both fashion of the time and the guard)
Blade length 27cm/10.5"
Grip length 9cm/3.5"
Overall length 38cm/15"