"Sky vessel" was the name, given by the Sheriff of Nottingham, to a spaceship that landed on Earth in the 12th century. It carried a complement of robot drones, observed by the Twelfth Doctor to have originated from the 29th century. It crash landed on while on a journey to the Promised Land.
It assumed the shape of a castle in Nottingham. By this point, the robots, masquerading as medieval knights, served the Sheriff of Nottingham, who was converted into a cyborg and aspired to take the English throne.
After the Sheriff was killed and most of the robots were destroyed, the remaining two took off, not knowing they didn't have enough gold content to fix their engines. As the ship would detonate and take out "half the country", the Doctor, Robin Hood and Clara Oswald shot a golden arrow into its engines, giving it the power boost it needed to reach orbit where it exploded. (TV: Robot of Sherwood)