Drume often summons one of mini bosses from Falcon Bastion (it is random, which boss appears):
- Grand Canon Dominius
- Grand Chaplain Gaunder
- Grand Commander Soeren
- Preceptor Lazare
It works like a typical summon - suddenly appear next to the Drume, deals damage, but as soon as you kill Drume, summon will disappear without leaving any loot or corpse.
I noticed that summons don't appear every time. It probably depends on time we spend on killing the main boss. If we kill Drume quickly, maybe he won't be able to summon his Falcon helper.