Dreg reavers are animated corpses of gargantuan creatures. They are all too common on Grixis
When a necromancer needs a heavy armor unit, he calls on the dreg reavers. These massive bestial juggernauts have the strength of elephants, the resilience of siege machines, and the damage-dealing capacity of a herd of baloths. Dreg reavers appear in a variety of shapes, all strong and horrible. They can be saddled with war platforms and used as siege engines, or simply set free to wreak massive destruction on a populated area. Some necromancers lash razor wire around the tusks, horns, claws and tails of dreg reavers, or attached barbed lances to their flanks to augment their pain-inflicting abilities.
"On our thirty-fourth day of digging, we unearthed a chamber that contained the intact remains of several species long extinct from Grixis. One in particular should make a fine siege engine . . . ." -Last notes of Shungus Nod, fleshcrafter