Oyerth

It's never really stated that the Ur-Didact died. It was just assumed. I wouldn't consider that a spoiler (or else it wouldn't have been in the official summary).

It is stated in Silentium's official summary that both Didacts are still alive. The Ur-Didact (the original Didact) is stranded in a Flood infested system while Bornstellar is with the Librarian.

I'm assuming you haven't read the Forerunner books by Greg Bear. Although it's not specifically mentioned how it's done (your mention of altering human dna), the Librarian can give Humans a geas - http://www.halopedia.org/Geas. (Note: don't click the link if you plan on reading the Forerunner books).