The advancing third-floor mobs that overran Brandon An as he covered the survivors' escape.
Shade Gremlins are the mob Brandon-An names in his dying message — the advancing horde he and the resident Henry held the line against as the third floor collapsed, buying time for Imani, Elle, and the others to reach the distant stairwell. Brandon 'hated those fuckers,' and his last request to Carl is to 'get them for me, brother. Get them all.'
Shadowy gremlin-like mobs of the third floor (undescribed in detail).
The mob that killed Brandon An — the target of his final vengeance-plea to Carl.