New bee swarm moved into an existing box

Last week I recovered an almost empty hive on one of the yards where I keep bees.
They clearly weren't in good shape and were reduced to one frame of brood and some honey.
I start feeding them with some sugar water in the hope to not lose the girls.
I thought they were doing better, with lots of activity.
But then suddenly today we heard a load noise, a swarm decided they were moving in.
I had no protective clothing on, 3 in my hair, 2 walking over my forehead when I made these movies:
didn't get stung once.