A pacified Viking warrior must pickup his sword and raise an army to rescue his son from an unlikely enemy hellbent on enslaving all of Scandinavia: unicorns.

I find that too many adjectives work against a logline – the specifics of each character should ideally be clear from the premise.

So as you suggested the fact that they kidnapped the boy makes them evil and there is probably no need to add it into the description.

Reverse the order of events to indicate the cause and effect:

After his son is taken by an unlikely enemy, unicorns, hell bent on enslaving all of Scandinavia, a retired Viking warrior must pick up his sword to save his boy and his people.