@JeremyECrawford Phantasmal Killer: Where is it the illusion-fear? it moves with target or caster chooses 1 point where it stays motionless?— Draconis (@DerynDraconis) July 13, 2017 Phantasmal killer creates an illusion in the frightened target's mind. The illusion isn't located in space somewhere. #DnD https://t.co/R8P2MPHcMO— Jeremy Crawford (@JeremyECrawford) July 14,…
@Pomp31us Does Truesight see through Illusions that are only in a target's mind, like Phantasmal Killer/Force or Weird? Thanks. DM's judgment, but I'd say no— Mike Mearls (@mikemearls) August 22, 2014
@JeremyECrawford @mikemearls Can the caster see Phantasmal Force? Can he move it or change it's behavior after casting?— Dale Everett (@dalebert11) May 19, 2016 Only the target of phantasmal force can perceive it. And the caster doesn't get to change it after casting. #DnD https://t.co/GvDIjqWEvr— Jeremy Crawford (@JeremyECrawford) May 27,…
But can that illusory 2x2x2ft bag move with the target, if the target moves beyond the 10ft area?
The 10ft cube is a constraint on the dimensions of the illusory object: “you create a phantasmal object […] no larger than a 10-foot cube”.
By RAW and JC’s Sage Advice, a suitable illusion (bag on the head, a fire in their pocket, a direwolf snapping at their britches) moves wherever the target goes because the illusion is in their head. It’s not Minor Illusion.
Lava flows and fog clouds are a different question. Silent Image can be moved as an action. Phantasmal Force lacks that wording, frustratingly.