In Morrowind, Dagoth Ur repeatedly mentions a promise that Nerevar made to him when they were both mortal. I've read multiple lore wiki articles and in-game books about Dagoth Ur, Nerevar, the Nerevarine, etc. but I can't tell what this promise is. The only promise I see is Dagoth Ur promising Nerevar to never use the Tools of Kagrenac or the Heart of Lorkhan.
I've found only one reference of a promise that Neverar made to Dagoth-Ur. It is on the www.uesp.net wiki, in the story about Nerevar at Red Mountain:
"Stay here, loyal Dagoth-Ur, until I return."
Here Neverar promises to return to the Red Mountain as soon as it is decided what to do with the Tools of Kagrenac.