What is Assise Of Mort D'ancestor?

Legal Definition
A real action which lay to recover land of which a person bad been deprived on the death of his ancestor by the abatement or intrusion of a stranger. 3 Bl. Comm. 185; Co.. litt. 159. It was abolished by St. 3 & 4 Wm. IV. c. 27.
-- Black's Law Dictionary