The belker can use its bonus action to assume a smoke form, much the same as the effects of the gaseous form spell. The belker must concentrate on this effect. A creature that enters the belker's space for the first time in a turn or ends their turn there must make a DC 15 Constitution saving throw, taking 18 (4d8) necrotic damage on a failed save, and half as much damage on a success. The belker can also move to another creature's space, triggering the Constitution saving throw.