Vehk's Extra-Dimensional Pit

An extra-dimensional pit opens on a point you choose within range. You can choose an area that is a 25-foot square or a 10-foot by 50-foot rectangle. The pit extends into a pocket dimension, so it can just as easily be created in the air or a body of water as it can on the ground. If the pit is opened under water it will only temporarily displace the water from the space it occupies. All surrounding water, including any above the pit, will stay in place magically. Any huge or smaller creature without a flight speed that is in or enters the area for the duration falls into the pit. Creatures inside can neither see other trapped creatures nor out of the pit itself, regardless of their size. Any other physical object not held or worn by a creature, including any terrain, will be unaffected; the pit is unable to displace it. 

Any creature that falls into the pit may use an Action to make a Strength, Dexterity, or Intelligence check (its choice) against your spell save DC. On a success, it is able to free itself from the pit on a side of its choice using half of its movement. Teleportation abilities and spells are unable to free a creature from this pocket dimension, though effects that would move a creature to another plane (such as the Plane Shift spell) are unhindered. If an affected creature gains a flight speed (such as by the Fly spell), it is instead easily able to free itself using 15 feet of movement.