The Grim 18th refers to an incident during the final turn of Spearbreakers that happened on the 18th of Felsite, 211 PS.
During some minor renovations tot he surface elements to the fortress, several moderately sized bands of Spawn warriors attacked the fortress. As many dwarves were busy working on the renovations and several soldiers were asleep or otherwise preoccuiped, the inital response was a desperate one. 2nd Squad and The Noble Hammers smashed into the inital onrush while Skitara and Saard attempted to hold the main gate alone, as the rest of thier squad was still gearing up and Dauros was being pulled back from fighting a forgotten beast since he'd accidently stored his pike due to a misread equippment order.
Due to the utter terror many dwarves felt, the attack near the courtyard prompted them to flee towards the Iron Barracks, right into the maws of the spawn who ignored Saard and Skitara. Those who were armed attempted to shoot the spawn down to varying results while others tried to flee. Several dwarves died, both military and civilian, in the ensuing chaos. Some fo the victims were founders Loud Whispers who was ripped apart and Splint who was clawed in the back of the head, Saard who died shortly after dispatching two spawn wih a scimitar she grabbed in her haste to head for the gate, Rose was struck down despite her best efforts, Gemblade was ripped to peices after falling ina pond, Telgin and Thornback being killed without much effort on the spawns' part, and Otiluke who was killed inside the very walls of the Iron Barracks itself, to name but a few.
The horrendous losses resulted in Simon Tam attempting to exercise more political authority, starting with claiming Splint's accomodations for himself, while Fischer, distraught over the casualties her forces sustained, asked the current Overseer Savero to put out a call for volunteers and rerganize the military to make things somewhat easier on the new recruits. It is believed by some that the simple presence of the post of Overseer may have prevented political turmoil in the fortress following Splint's death.
Volunteers flooded the reorganized forces and they were all quickly jammed into the training simulator to get them up to snuff and avoid a similar disaster. The first fo these reorganized units to see action were The Morningstars lead by Sergeant Kreia, who defeated a scouting force sent by The Empire of Confederacy not long after.