RAW QUESTIONS / FAQ / EVENT LOGISTICS
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It's a jumble of explanations and difficult to get a more complete answer. Rather than link a bunch I'll post a TLDR of my searching. Technically you don't need to go further than the PHB to have a solid ruling, but there are a number of JC and Sage Advice posts regarding this which are good to read. I feel they support the following when all is considered.
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From the PHB:
PHB 204, A Clear Path to the Target: To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
PHB 196, Cover: A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle. (THIS LAST BIT IS IMPORTANT!)
PHB 196, Cover: A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
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The real question is regarding targeting and the origin of the effect. Wall of force ONLY says "Nothing can physically pass through the wall." So it grants (let's call it) total PHYSICAL cover, but since it is transparent, it does not grant concealment. The debate around total cover doesn't acknowledge this much, at least I couldn't find references, and this is actually key, as is how the effect originates.
Concealment is a required element for total cover, and an invisible wall does nothing to hide Strahd. The test? Were he to attempt to hide, it would automatically fail without mitigating circumstances, regardless of having total PHYSICAL cover. So I don't think the WoF grants total cover, and as such, doesn't prevent targeting. As for the physical cover, any spell effect that travels from caster to target will be stopped by any physical obstruction, so the WoF gives Strahd a version of total cover, just not one that prevents ALL targeting.
Examples:
Misty Step - "Target: Self," and only requires you to see the spot you wish to go to. It makes it through.
Fireball - Targeting doesn't matter as Fireball does not require the point to be visible, "Target: A point you choose within range." What matters is the Clear Path to the Target. Fireball will hit a wall if it is in the way, so it is stopped by the total PHYSICAL cover of WoF which will block both the initial streak as well as the aoe effect, which is also physical. It gets blocked.
Sacred Flame - "Target: A creature that you can see within range" and "The target gains no benefit from cover for this saving throw." The second one is distinctly important because it doesn't refer to targeting, just to the actual saving throw. This means any benefit cover might give, like a +2, is ignored. The target is not concealed and can be seen, therefore they do not have total cover and can be targeted. The spell effect does not travel but is generated at the target and its not physical, so it ignores the WoF. It makes it through.
Charm - "Target: A humanoid you can see within range." If we go by the litmus test of spells before this, then... Strahd can see us, so we aren't concealed and thus, do not have total cover and can be targeted. There is no physical effect that travels or is even generated. Charm makes it through.
Thoughts?
It seems clear to me that charm can go through the wall of force, because it is not a spell, nor is it an attack (an "attack" is always defined as a d20 roll vs. AC).
The moderator of the RAW facebook group points out that the rules are silent on how effects that are neither spells nor attacks interact with full cover. Thus it falls to the DM to decide.
The charm is a creature ability, not a spell or attack, and the ability clearly states that he just has to "see" the target. The RAW is pretty clear in this case that the charm should work, for the same reason that misty step can go through a wall of force: specific wording (the ability description requiring sight of the target) overrides general (the wider rules about cover, etc.).
Of course Gash the Good merely need to shout out to the group, "avert your eyes from his gaze!" }:-)
Update on Wall of Force:
So we have definitely been doing Wall of Force wrong. Here is a quick summary:
All spells require an unobstructed path from caster to target.
https://www.dndbeyond.com/compendium/rules/basic-rules/spellcasting#AClearPathtotheTarget
To target something [with a spell], you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
Spells that say "that you can see" require the caster to be able to see the target, in addition to an unobstructed path.
If the spell does not say "that you can see" then there is no requirement for the caster to see the target. They still need the unobstructed path.
The rules are silent about effects that are not spells, such as class features, monster stat block actions, lair actions, etc. These effects do not have the same requirements, so if they say "a target you can see" or something similar then they can go through a wall of force, depending on the specific wording. A good example of this is a vampire's charm or a cleric's Turn Undead feature, both of which could go through a wall of force.
Short answer: Jeremy Crawford's tweet on the subject makes both the rule and the intent behind the rule clear.
https://twitter.com/jeremyecrawford/status/803404321484247040
"A solid obstacle, regardless of material, can provide total cover. A closed window counts."
Thus, no spell effect can penetrate a wall of force unless it can ignore cover, or can go around it somehow (example: dimension door/teleport/sending). You definitely can not counterspell through a wall of force, for example.
