Edit: i also googled it and couldnt find anything about summons being taken over by conjurers, so maybe its not commin knowledge?
Let me extend Greetings,
Welcome to the Bethesda Forums! (Nice to meet you, I'm King.)
In what you are saying, the only similar experience I have had is even Magnus turned my Frost Atronachs against me.
You can do the same to their summons by using the spell Commanda Daedra.
http://elderscrolls.wikia.com/wiki/Command_Daedra
This... why I don't rely on deadra conjures at really high levels.... It is as it happen's really fun to use their summon's against them...
Fie, for the Conjurers, they will need to conjure up a grave for themselves!
From my personal experience:
Dragonpriests, Thalmor-Mages and Conjurers do use the command-daedra-spell.
However, they do it only up to frost-atronachs. Storm-Atronachs and Dremora-Lords are safe.
That means for me only: Do not let your char attack an Dragonpriest until you can conjure Dremora-Lords and Storm-Atronachs!
I've had Krosis take Frost, Flame and Storm Atronach's... on 360, so vanilla..
Interesting!
At which level of conjuration?
Did you have the 80th-perk for potent-atronachs?
I want to say no...cause by the time I would get that perk, I would have tackled Krosis already.. I'll have to try it.. soon as I get a character that will get conjuration high, none of the characters I have now will.