Vikings face just one alien in the film
Outlander, which reimagines
Beowulf as about a human from another planet hunting a dangerous and wily alien on Earth in the Vendel era.
The
Predator fan film
Dark Ages runs with the films' suggestions that the Predators have been visiting for a long time by setting one against a group of Crusaders.
For a whole bunch of aliens, see the comic miniseries
Lake Of Fire: "It is 1220 AD, and the gears of the Albigensian Crusade grind on. When an alien spaceship infested with a horde of bloodthirsty predators crash-lands in the remote wilderness of the French Pyrenees, a small band of crusaders and a Cathar heretic are all that stand between God's kingdom and Hell on Earth."
The aliens are very
Alien style bugs, with the notable difference that they don't have acid blood so that fighting them hand-to-hand with swords and axes while wearing metal armour isn't totally hopeless.