Incredible Spell. Can be used in Silence as it doesn't require any Verbal component.

Probably one of the better Area of Effect control spells that can be used with Silence.

Do you get advantage if attacking an incapacitated creature?

I don't believe so, not RAW anyway. In the Player's handbook it only has the following:

• An incapacitated creature can't take actions or reactions.

If they become prone because of it then yes.

Do creatures with Truesight simply ignore this spell?  While it is not a typical visual illusion in the sense of invisibility or magical disguise, it still is a visual effect and does belong to the illusion school..

Creatures with truesight will most likely have immunity to charmed, but still an interesting point to make

This spell is really good, and when used in conjunction with the Bard item Doss Lute https://www.dndbeyond.com/magic-items/doss-lute it becomes OP bordering on broken as it makes all targets roll Disadvantage.

Amazing for nullifying a room. Everything a 30 ft. cube makes a Wis save or can't act, react or move. Nice.

Awesome against most basic mobs, as they won't have high Wisdom.

And for DM's, this is brutal against the party's tanks. Most of them won't have proficiency in Wisdom saves (glares at the Paladin), letting your illusionist BBEG stun them all into a stupor to get away, or to neutralise them for long enough for a more permanent solution...

If I cast Hypnotic Pattern in normal darkness, does it have an effect? Can targets see the colours? The spell description notes that it generates colours, but not light.

If I cast it in an area affected by a Darkness spell (Level 2), does it have an effect?

“Each creature in the area who sees the pattern” so unless they have a way to see through the given type darkness then no, it wouldn’t work.

Can you interrogate someone under Hypnotic Pattern since they are still charmed?  Like can they respond to you verbally?

No, because they are incapacitated. 

doesn't say it, doesn't do it

I know you're an inadequate creature. I was wondering if Targets can see colors? 

Given that "stunned" is "incapacitated except you also can only speak falteringly" (plus a few other things), that suggests that you can respond verbally while incapacitated. However, note that Hypnotic Pattern does not contain the "charmed person considers you a friend" rider from Charm Person, so your checks might be quite challenging.

That does sound more correct - as if you look at petrified, it specifically says that you are incapacitated AND unable to speak.

And given the incapacitated "icon" i think it just means "your mind is fuzzy"

i think i'm getting old.
Hypnotic Pattern, Banishment, Counterspell, and Heat Metal all got nerfed in my games. Some spells just break the game, and if many different players across many different games consistently all seem to use the exact same spells to trivialize almost every fight, it's probably a good idea to examine why.

Anecdote: I recall my first time playing D&D. I played a wizard in the 3.5 system. I cast Hypnotic Pattern behind a partymember, assuming the pattern would occur behind its back and not affect her – wrong. She got incapacitated and killed off :(

Yea, it's pretty fair to nerf all of those spells. How do they work when you change them around?

It says they are Charmed (you have advantage on social checks against them) and Incapacitated (can't take actions/reactions), but talking doesn't take actions, so I think they can still talk. They aren't Stunned or Petrified or anything... they can still talk, then! This is actually a really good spell for interrogating creatures with low WIS saves.

Casting Spellwords are:

Anodyne Exemplar