Diabolus ex Machina is the inverse of Deus ex Machina. Deus ex Machina have plots miraculously resolve in a fashion that breaks suspension of disbelief, is not foreshadowed, and/or just looks like really lazy writing. The inverse is when the antagonists pull a fast one on the audience by calling in reinforcements, pulling out a new power, or otherwise stretching out the narrative after they should have already been defeated. Alternatively, a Diabolus ex Machina can just be the hero dying at the end of a tale for banal reasons.
Diabolus ex Machina are subjective just like their good counter part but suffer to an even greater amount of obfuscation because unlike the Deus ex Machina we don't expect to have as full an understanding of the challenges/threats/opposition as we have of the protagonist and his allies and resources. The slow doling out of information around the core conflict in a plot is a huge element in establishing rising tension on the Doylist level and it is unrealistic for the protagonist to enter the plot with perfect knowledge on the Watsonian. Because of these tendencies Diabolus ex Machina are really hard to pin down in most works.
IRON NODER X: XTREME XCELLENCE