It is a permanent event in which at a certain time on a certain day, a group of Vikings besiege our settlements, so we have to defend our buildings.
These Vikings are very hard to bring down, and if we succeed, their bodies may obtain a part of their team, armor or weapon. But only one of them.
For example we are attacked five Vikings then one will have to recover the object.
This attack Viking last a short time and their power of demolition, although very powerful, not give them time to destroy everything and yet be coordinated very well the defense if we kill them all and so tner the opportunity to get valuable booty.
If last time "determined" not killed ... they will leave having destroyed all that we can not defend ...
I think that will help the group cohesion, coordination and especially when tension is the attack and its substantial reward. Reward to be balanced by the creators and objects that are a bit unusual.
I hope you like my proposal!