Lord Smedth is a Deyjan lord that's mentioned in the With Blinders On scenario of the Specter of Power campaign in Heroes of Might and Magic III: The Shadow of Death.
When Sandro had completed his preparations for what would become the Restoration Wars, he learned from King Finneas Vilmar that there was one Deyjan lord that opposed his plan - Lord Smedth. Sandro decided to take care of the upstart himself, as Smedth had been a "minor thorn" for some time, always harassing him in court and attempting to take his place as Finneas Vilmar's closest advisor.
Sandro went out and destroyed Smedth and his allies, but when he returned to court, Finneas had him imprisoned for "assaulting an innocent lord". This would allow Finneas to rule Deyja alone, and he could steal the credit for Sandro's plans to invade Erathia.
There are no heroes named "Lord Smedth" in the scenario. The last hero Sandro encounters is called "Lord Dufus", but is referred to as "Lord Amrothal" at the start of the scenario. It is not known of these names are meant to refer to the same individual.