Fligating IM destroys creatures that enter after their capability has been resolved.
The primary related rule is Rule 611.2c:
If a steady impact generated by the decision of a spell or talent modifies the traits or modifications the controller of any object, the set of objects that impacts is decided when that steady impact begins. After that time, the set won’t change. (Observe that this works otherwise to a steady impact from a static capability). A steady impact generated by the decision of a spell or talent that doesn’t modify the traits or change the controller of any object modifies the foundations of the sport, so it might have an effect on the objects that weren’t affected when that steady impact started. If a single steady impact has items that modify the traits or change the controller of any object and different components that don’t, the set of objects to which every occasion is utilized is decided independently.
The primary a part of Maddening’s skill is a steady impact that doesn’t change the traits or controller of any object, so it might have an effect on objects that weren’t affected when the capability was resolved. The delayed triggered capability is just not related to any explicit set of creatures when it’s created, so it determines the set of creatures to destroy as it’s resolved within the ultimate step.
That is confirmed by selections in This Reddit thread.
