I liked my post in
this thread enough that I figured it would be helpful to post it over here where people actually looking for Tempest Cleric information might actually find it.
The error you've made here is completely understandable. I can definitely see, just reading the sentence in the spell, that you think it ordinates from you and spreads around you. However, that is not how the spell works. You have to refer to the Player's Handbook, page 204, the section titled "Areas of Effect".
There you will find the entry for Cube. That entry says, "
You select a cube’s point of origin, which lies anywhere on a face of the cubic effect. The cube’s size is expressed as the length of each side. A cube’s point of origin is not included in the cube’s area of effect, unless you decide otherwise."
So you know it has to be a cube, and it must originate from the caster, and the origination point must lie somewhere on a face of the cubic effect. So the caster must be in a space adjacent to a face of the cube to serve as the origination point and it spreads into a cube from that point.
Here are some legal options for that (the caster is the asterisk):
Again, I can definitely see why you read it the way you did. But, it doesn't quite work that way. Perhaps it's not very intuitive the way they wrote it.
And, as a "Damage Type: Thunder" spell, as a Tempest Domain cleric starting at 2nd level when you roll lightning or
thunder damage, you can use your Channel Divinity to deal maximum damage, instead of rolling. Which has several advantages. First, since Thunderwave auto-hits (though the target gets a saving throw for half damage) you can use this ability on ALL castings of Thunderwave, as long as you do any damage at all. Second, the spell scales. So it does 2d8 at first level, and then an additional 1d8 for each spell slot used after first. So if you use a 3rd level spell slot, it does 4d8 (save for half). If you burn a Channel Divinity use with a casting, that spell does 32 hit points damage and pushes foes 10 feet (or 16 damage on a save). Now compare that to an average fireball, which is 8d6 (save for half) or 28 average damage (or 14 on a save).
That's a pretty sweet deal for the Cleric - who isn't a fireball type caster, and isn't typically using area effect spells to begin with. They're doing more damage on average to each target, and though it's a smaller area of effect, it also pushes targets who miss their saves (and the save is a Con save, which is less common than the dexterity save of fireball).
And then you have Shatter spell, which does 4d8 thunder damage as a 3rd level spell, which maximizes to 32, or 16 on a save, with a 20 foot diameter sphere and can be cast anywhere in 60 feet of you.
And then Call Lightening, which can be truly devastating, and last a long time. Cast that sucker during an existing storm and you're maximizing 4d10 as a 3rd level spell every round for up to 10 minutes!
And you get Glyph of Warding, which is Thunder or Lightening Damage, either of which you could maximize. Or you could just store a Thunderwave in it and maximize that. Either way, carry a few blank 10' cube flags with you, cast it during your short or long rests on the flag, and make the trigger "an evil creature comes within 5 feet". Now all you need to do is toss that sucker down and you've blocked an entire 10' wide hallway which does 5d8 thunder (or half on a save) in a 40' diameter sphere (which is friggen huge) which you can maximize to 40 damage (or 20 on a save). That's like a fireball cast in a 7th level spell slot (and it's also similar to delayed fireball) from a 3rd level spell slot!
You can even use this ability with one of your other Tempest Domain abilities. Wrath of the Storm, an ability you get at first level, let's you use your reaction to do thunder damage to a creature in melee range that hits you (2d8 thunder damage, or half on save). You can use that same Destructive Wrath ability to Channel Divinity and "When you roll lightning or thunder damage, you can use your Channel Divinity to deal maximum damage, instead of rolling." So you can automatically do 16 thunder damage, or 8 on a save, to a creature that hits you, as a reaction (and reactions are rare for a cleric).
Your 8th level ability Divine Strike also let's you do Thunder damage (1d8 or 2d8 depending on your level) which you could also auto-max with a use of Channel Divinity.
You can also pick up the Magic Initiate feat and grab cantrips like Lightning Lure (which lets you pull a creature 10 feet toward you, then do 1d8 lightning damage to it or 2d8 at fifth level, which you could choose to maximize, and then your 6th level Thunderbolt Strike ability lets you push them back 10 feet again. And boy, if the wizard cast a wall of fire or something else nasty in that area you've pulled and pushed them back through, look out!). Shocking Grasp, Booming Blade, and Thunderclap are all also good choices for those two cantrips you'd get. Chromatic Orb, Thunderous Smite, and Witch Bolt would all be good choices for the 1st level spell you'd get.
And unlike the "save for half" issue with many of the spells i've mentioned, if it triggers on a hit like it does with things like Booming Blade, you can choose to use the ability or not, so you'd only use it when it's most effective.
You don't even need the feat though. You could pick up one of those cantrips, like Booming Blade, by just being a High Elf.
Speaking of races, you could choose a lightning type for Dragonborn and get a breath weapon you could maximize.
Note also the two domain abilities we're talking about most here are not specific to CLERIC spells or damage of a thunder or lightning nature. It's ANY damage you do of those types:
"Destructive Wrath: When you roll lightning or thunder damage, you can use your Channel Divinity to deal maximum damage, instead of rolling."
"Thunderbolt Strike: When you deal lightning damage to a Large or smaller creature, you can also push it up to 10 feet away from you."
Which means you can gain one or both of these abilities and multiclass and use those abilities with the multiclass spells and abilities you gain which does lightning damage. You could do 320 damage with chain lighting (split between four creatures) for example (and knock them all back 10 feet)! A single lightning bolt is doing almost 50 damage to each creature in a line as a 3rd level spell (more at high spell slots). Chromatic Orb seems a good choice as well (and I think you could twin that as a sorcerer, while maximizing it with your cleric ability?). There are some REALLY devastating potential multiclass options there. I've seen Storm Sorcerer is a popular multiclass choice for a Tempest Cleric, due to the consistent theme. A Dragonborn (Bronze) Tempest Cleric/Storm Sorcerer wearing plate armor and shield and wielding a warhammer and specializing in Thunder and Lightning spells would be rather thematic and potent!
And if you find a magic weapon which also does lightning damage (like a Javelin of lightning, Seeker Dart, Staff of Thunder and Lightning, or Windvane)? Oof, knock foes back 10 feet on every hit baby!
The sub-class has a lot of synergy in it with that Channel Divinity ability. Being able to maximize damage as a non-sorcerer is pretty special.
