While it can be easy to see how the Great Old One Warlock's Thought Shield feature interacts with spells that deal psychic damage or special attacks from psionic creatures, I'm not sure how to rule "second-hand" damage.
The relevant portion of the Thought Shield feature reads (PHB, p. 110):
You also have resistance to psychic damage, and whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do.
Here, my question lies in what the phrase "creature deals" is meant to be interpreted as. In my mind I'm thinking it should read closer to the Undying Warlock's Among the Dead feature (SCAG, p. 139), saying something like:
If a creature targets you directly with an attack or a harmful spell, or if it includes you in an area effect, and deals psychic damage to you, that creature takes the same amount of damage that you do.
Is it just that the latter is too wordy / rules lawyer-y? Or should premeditated damaging effects like traps, Glyph of Warding, summoned creatures, or spells cast through dummies (such as Trickery Cleric's Invoke Duplicity) have damage dealt to those who set them up too?
My group is running a multi-character Player-vs.-Player storyline that I'm helping DM, and I'm trying to get ahead before issues with our GOO-Locks crop up.