Stinking Mud was a Earth2 and Water2 spell back in FE. Incredible spell, probably overpowered, because the AI would walk right into the mud and get stuck. If you brought archers or mages, they got to take their time ripping the enemy apart.

Like Primal wisely reported, you have to summon a crag spawn in tactical to cast it now, or get the Tectonic bulwark.

Also, are things working as intended in that you don't get the weapons that monsters are wielding when you defeat them? I just killed a coal golem and didn't get it's awesome pyre brand.

Probably for the best, that sword is so good it's almost a win button.