The spell goes off at the start of Xander's turn.
On the far side of the room the ground shudders and animates in a pile of loosely connected stone. It lashes out at the Derro facing Milo.
A Small Earth Elemental appears two squares north of Milo.
Tremor Sense ignores miss chance from darkness
Slam, Power Attack, Flanking, Earth Mastery: 1d20 + 6 - 1 + 2 + 1 ⇒ (20) + 6 - 1 + 2 + 1 = 28
Bludgeoning Damage, Power Attack, Earth Mastery: 1d6 + 4 + 2 + 1 ⇒ (4) + 4 + 2 + 1 = 11