Most likely the didact will just unleash his vendetta against humanity (as the only thing that prevented him from not wanting to kill humanity was the librarian, his wife)
but with the librarian either dead or somwhere else in the universe (no comfirmation until silentium comes out)
the didact is left, the last of his species, with nothing to do BUT vent his anger on the fact the humans were the intended inheriters of the mantle, how the humans defeated the flood, and how they are the forerunner’s reclaimers.
It could be a act of jealousy or just a grudge (the ur-didact was the one who faught and defeated the ancient-advanced human genus’ in the war)
and remember guilty sparks quote “the ancient enemies returned” in the halo CEA terminals; by “ancient enemies” guilty spark was referencing the ancient humans
he could’ve somhow contacted ur-didact’s cryptum via the domain while on delta halo (installation 05) and/or the ARK