The only thing like this I can think of - with the 18 number - happened during the Carolinas campaign and had absolutely nothing to do with Forrest. In November of 1862, Forrest was preparing to retake Nashville and was in middle Tennessee with a newly trained and organized brigade - this didn't happen so he embarked on his West Tennessee Raid. No killing of any prisoners in November because he didn't have any to kill!
However, much later Kilpatrick discovered 18 Union soldiers had been killed in two separate incidents and wrote Wheeler that he'd burn out everybody in his path if some explanation wasn't forthcoming, while Sherman wrote to Hampton demanding an explanation of exactly what had happened and he'd better be hearing from Hampton promptly as he intended to kill a like number of Confederates in retaliation. Hampton replied - truthfully as it came out later - that he didn't know what happened to the Federals but that Sherman shouldn't be surprised if somebody killed his 'foragers' during their depredations. Sherman answered Hampton shouldn't be upset - foraging was as ancient as war and a right...Well, Hampton didn't see it that way. He promised to kill two for each one Sherman killed and the whizzing war was on. I'm not exactly sure how this resolved - I believe Sherman just didn't kill anyone, which would have set off a most unpleasant exchange. Hampton meant what he said and didn't back down from much of anything.
As to Forrest executing prisoners, there's only one rather murky incident of that and that was during the Murfreesboro raid. At the time of the raid there were two scouts and two civilians accused of spying in the jail, sentenced to be executed. That event was interrupted by Forrest's raid, but the soldiers attempted to carry out their orders by shooting through the bars of the door. The four inside ran up to the wall and the soldiers couldn't lower their rifles enough to get them, and left. However, one came back and lit fire to the jail. After most of it was over, Forrest asked the men if it was true someone had tried to murder them. Yes, it was and the guy was present. "Point him out to me," said Forrest and one of the prisoners did. Later, there was a role call of the Union prisoners and this fellow didn't answer. "Pass on, " said Forrest, and that was all.