Mission Letter from Delilah is a note found in Dishonored 2.
Transcript
Mariah,
Take Heather and Kai. Travel to Karnaca tonight. The others there will give you a roof to sleep beneath and they will keep you out of sight. Watch our friend from Dunwall.
When the moment comes, strike. If you succeed, I promise you will be rewarded. And beyond that I'll make you a portrait.
-Your Empress Forever, Delilah
Location
The note can be found on one of the witches near the end of the mission The Clockwork Mansion.