I got the impression that the Rending begins the moment an Epic gets their powers, and is compelled to use them to their full extent, in the most destructive ways possible. That's the beginning, when the corruption is allowed to take root. I think gifters can resist the corruption after that, the way Prof did, but the corruption still sets in. The more power used, the more corruption sets in and the stronger it is, as we see with Megan post-resurrection.
With Euphoria, I think she'd definitely use her powers to bend people to her will, similar to how Quota uses his. Only in Euphoria's case, she'd create legions of happy slaves, whereas Quota just makes his victims too terrified and/or depressed to flee. Her Rending could have been extremely creepy, what with her dosing vanillas with high levels of happiness only to cheerily murder them.