Iliana Ghemor really gets put through the wringer in Fearful Symmetry.
After her childhood sweetheart gets killed in a terrorist attack on Bajor, she begrudgingly joins the Obsidian Order. During her training, she is deceived into killing someone in what she thought was a training simulation. Then, after she gets assigned a mission to infiltrate the Bajoran resistance and undergoes a surgical procedure to assume the identity of Kira Nerys, Dukat takes her captive and forces her into slavery for years. When she finally manages to escape at the tail end of the Dominion War, she regains her memories as Iliana, but retains her memories of her undercover identity as well; the two warring identities drive her mad.
Pretty dark stuff.