That's the common myth. The event it refers to is the battle of Fort Pillow in Tennessee. The mythical version claims the fort was either all black or majority black, that it tried to surrender, and that its people were mowed down in brutal slaughters for hours after the battle. There isn't any credible record to support those claims though and most were made up for propaganda purposes to boost recruits into the northern army on an outrage emotion.
What really happened at Fort Pillow is this: Forrest's vastly larger army came upon a small yankee-held fort that was about half white, half black. During some initial skirmishing virtually all of the fort's superior officers were killed, though this was unknown to Forrest at the time. A ceasefire ensued in which Forrest sent a message to the fort's commander (who was deceased, unbeknownst to him) saying the situation was hopeless and that they should surrender. A hotshot lower level officer had since taken command even though he was inept and unqualified to do so, but rather than respond to Forrest's request he forged a message back to him under the name of the dead commander informing them that they would stay and fight. Forrest then ordered the attack, which was swift and bloody as the fort was easily overrun. Barely a moment into the attack the remaining yankee officer command collapsed entirely, with low level officers giving simultaneous and contradictory orders to form and hold a line, to surrender, and to retreat to the river. This produced a situation where men standing side by side were following opposite orders. Some even laid down their arms as ordered only to pick them up again and resume the fighting when they saw people next to them getting different orders. It was a brutal mess in which the yankees sustained heavy casualties - primarily as a result of disorganization. In the heat of battle a few cases of brutality were reported, though that's about it. Forrest and the other officers are also recorded to have arrived on the front line at the end of the battle and specifically put a halt to the soldiers who were out of control.
Thanks, I will accept you account of what happened.