I don't know what the D&D5 rules are on relativistic time dilation, I guess these would perhaps need to be invoked.
- if the rod travels across 7 light seconds in a round, the only way to avoid breaking relativity is if the 6 seconds the round takes are measured in the frame of reference of the rod
- that would mean that from the frame of reference of the rest of the people/monsters/edgy antiheroes/misunderstood blob creatures, the rod’s “turn” took 7 seconds.
All characters in the D&D universe are accustomed to a reality where each round takes 6 seconds, and everyone - in synchrony - is able to perform an integer number of tasks that fit within that timespan. Rounds begin and end simultaneously for everyone involved in combat. How disturbing would it be for such beings to see those laws broken?