The Great Diminishing

The Great Diminishing refers to a span of time --some say decades, others say millennia-- in which the amount of ambient magic in which the influence of the Primordial Titans (giants, dragons, and leviathans) ebbed away and made room for the domains of men.
Scarcely little is known about this time in history. The term is derived from an event that is recorded in the annals of giants and the retellings of dragons. Both sources describe a diminishing of not only their power and influence, but also their physical size and strength.
Geological evidence that's been scrounged up through the years indeed points to a time when these Titans were indeed several orders of magnitude bigger than their modern iterations.
What caused this phenomena is ultimately a mystery, but the recordings of the giants and dragons both point the blame at a so-called "great deceiver" who distracted them from whatever machinations brought about their decline.
The giants name this deceiver "Vicium," while the dragons called him "Tahrovin." Throughout history and legend, other trickster spirits in the same vain have popped up under names like Nihil, Tul'mith, Jekenheil, or even Asmodeus. Modern sages have a much more succinct name for him these days, however: Cataclysm.