As it still stands out, hydroid's plunder does not seem to work properly in a lot of cases. It is completely possible to stand next to tentacle swarm tentacles that hold up multiple enemies and regardless of how many times you cast plunder, you will not get anything in return. This has been an issue since day 1 of the rework and continues to plague otherwise quite a good rework and it is really getting tiresome how badly this ability works because of the line of sight check.
Plunder works p