The warlock's Cloak of Flies eldritch invocation (XGtE, p. 56) says:

As a bonus action, you can surround yourself with a magical aura that looks like buzzing flies. The aura extends 5 feet from you in every direction, but not through total cover. [...] Any other creature that starts its turn in the aura takes poison damage equal to your Charisma modifier (minimum of 0 damage).

The gaseous form spell says:

You transform a willing creature you touch, along with everything it's wearing and carrying, into a misty cloud for the duration.


While in the form of a misty cloud, [...] The target can't attack or cast spells.

A warlock activates the Cloak of Flies invocation, optionally casts armor of Agathys, then casts gaseous form. Cloak of Flies is not a spell, does not have an attack roll, and persists until dismissed. Therefore, it should do damage to any other creature within 5 feet even when the warlock is a misty cloud.

Can the warlock expand the misty cloud from gaseous form so as to damage multiple targets? If so, how large an area could be covered?


No, gaseous form can't expand the aura of Cloak of Flies

Spells do what they say. Gaseous form treats the misty cloud as a creature, as evidenced by the fact that the spell ends

if the creature is reduced to 0 hit points

and nothing in the spell changes the target's size from Medium to Large (or larger), or allows the creature to do so. Therefore, the Warlock in misty form is still a Medium creature, is considered to control a 5'x5' space, and the Cloak of Flies aura still extends from that space - i.e. no change in the area affected by the Cloak of Flies invocation.


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