Unfortunately I was not quick enough to make screen shots while been murdered, so here are a few of post-attack:
Our bodies and alts running in panic around them:

That is how they got in (yes, we missed that one):

Supposed base of operations, it appeared a few days ago and we were on alert because of it focusing our attention on blocking cave entrances to our village:

We had a little layout of sceletons under the charter stone to make it look like a sacrifice altar or a ceremonical burial place, they extended it with one more sceleton:

The last one is weird, seems they summoned using charter stone. Is that possible?
So who are you, our dear attackers? And why have you done it? At glance most things are intact except some clothing on dead bodies. Or was our stuff not nice enough for you to take?

EDIT:
Perpetrators were tracked to this base:


...at this location:
http://algeralith.info/HavenMap/?x=-28&y=18&zoom=9