I think prior to season 6, the Klingons and Romulans tried to destroy the Dominion by taking out Odo's homeworld in a sneak attack. Odo's people turned out to be the rulers of a large empire that sought to conquer solids (non-shapeshifters).
Re:Odo: Shapeshifting for your data
by moosingin3space on Tuesday January 03, 2017 @07:07PM
(#13314505)
Cardassians, not Klingons.
Re:Odo: Shapeshifting for your data
by cholantesh on Tuesday January 03, 2017 @09:47PM
(#13315272)
Also when Odo was forced to return to the Link to be judged (end of season 4).
http://memory-alpha.wikia.com/wiki/Odo