Ground Tentacle is a ranged monster skill used by the Death Mauler class monsters.
The Death Maulers will thrust their hands downward and project their tentacles into the earth through which they will quickly burrow toward the target before shooting up in two deadly spikes. Moving dirt mounds can be seen as the tentacles travel, giving advance warning of their approach.
In groups, these spikes can be quite devastating, so it is best to destroy the monsters as quickly as possible. As the tentacles will not change target after they have been injected into the terrain, it is also wise to avoid remaining in the same place for very long while fighting these creatures (if they are in view, you should move only after they begin leaning over to use this skill, as at that point, the target is locked).