Those are grave elementals, warped earth elementals that dwell in and haunt graveyards. They are very hateful and aggressive monsters that pound any living being they come across into the ground.
They possess a powerful tremorsense enabling them even to detect invisible things, and can effortlessly swim through earth and stone.
Their most horrible ability is the ability to sink their foes into the ground, summoning bony arms to pull them into an early grave where they lie in suspended animation until someone excavates them. This seems to be what is happening to Karlat right now.