Sister Svangerd and the Not Quite Dead by K.J. Parker

Not even the Church of the Invincible Sun is invincible - and somebody has to do its dirty work. Enter Sister Svangerd and her accompanying priest, both accomplished practitioners. Their mission is simple: to make a meddlesome princess disappear (permanently).

To get to her, they must attend the legendary Ecumenical Council, the once-in-a-century convening of the greatest spiritual minds the world has to offer. But when they arrive, they find instead a den of villainy that would make the most hardened criminal blush.

To complicate matters further, it appears that some people long considered dead might not be after all. What began as a simple murder is about to escalate into a theological debate of terrifying complexity.