Yeenoghu Casts this at the 8 th level.  Some flying player at least for one round going to have to take 4d8 damage or as the kill shot for the melee who got paralyzed by the butchers mace

Not quite. Just one attack on the first round, which is part of the (Bonus Action) casting of the spell. You may still take your regular Action to make your own weapon attack or cast a cantrip. (Or various other less common things)

Your subsequent round interpretation is correct, except you don't have to take the Bonus Action after your regular Action. You can take your actions in any order.

Gruns

Am I right to assume the attack roll for this weapon gets the cleric's magical to hit bonus?  e.g. 5th level cleric would apply its +6 spell attack modifier to the attack roll from this weapon.

Additional bonus question, does this weapon count as a magical weapon?

So.. I'm thinking this would actually work as an outlet for the stored energy of Absorb Elements? The SW is a melee spell attack but should still count as a melee attack, even though it's not near the caster.

Alright, weird ask. I'm playing a Divine Soul Sorcerer. If I twin this spell, can I only use one at a time, or do both use the same bonus action or should we simply use one with an action and the other with a bonus action?

You can't really twin this spell since Twinned Spell requires a spell that targets only one creature. Spiritual Weapon isn't eligible, since it can be cast without a target.

Additionally, a weapon swing requires a bonus action and you have only one of those per round. Else you could just spend multiple spellslots to fill the area with Spiritual Weapons since it's not a concentration spell. The spells' power is balanced around the one bonus action.

edit: this has already been answered. 

but I do have a question: does the spiritual weapon get attacks of opportunity? 

Bonus action from the "Combat" section under "Your Turn":
"You choose when to take a bonus action during your turn, unless the bonus action's timing is specified..."
Attack from the "Combat" section under "Making an Attack":
...the rule is simple: if you're making an
attack roll, you're making an attack.

If you only have one attack per round, you must choose to attack with the spiritual weapon or do something else. It would be called, "Spiritual Attacker" if it were otherwise. You get the bonus action to move the weapon. You would be able to move, as well.

Quote from DMCubed >>

edit: this has already been answered. 

but I do have a question: does the spiritual weapon get attacks of opportunity? 

 I have the same question; does it?

Quote from DMCubed >>

but I do have a question: does the spiritual weapon get attacks of opportunity? 

 The Spiritual Weapon does not get AoO, because it's not a creature.

You cannot use it to to make an AoO because the spell specifies that it requires a bonus action to make an attack. An AoO is done with a reaction.

Can the second attack be against the same target?

It doesn't make opportunity attacks, so that makes me think it's hardly there and folks can run through it.

Some bad spell casting rules posted here.  For clarification, if you cast a spell using a Bonus Action (such as the turn you cast this spell) then any other spell you cast that turn can only be a cantrip with a casting time of one action.  However, as long as they're not Bonus Action spells you CAN cast more than one spell (cantrip or not) in a single turn.

Examples of this:

• Casting a spell with a casting time of one action, then using Action Surge and casting another.
• Casting a spell with a casting time of one action, then casting something as a reaction, such as Shield, Feather Fall, or Counterspell (if someone tried to counter your first spell).

You could even use a reaction in that Action Surge example, for three spells in one turn.

I interpret this to mean that in round one it is cast with the bonus action and then the action is used for a melee spell attack, in subsequent rounds the action and bonus action can be used to make separate melee spell attacks within 20 feet of each other. It is powerful, but it looks like a way to make up for clerics not getting a second melee attack, it's also limited by that 1 minute duration making it useful for just a single encounter per casting. 

My cleric uses a simplified symbol of Lathander as his spirit weapon, which happens to closely resemble the Krull glaive.

Quote from Black_Chocobo >>

I interpret this to mean that in round one it is cast with the bonus action and then the action is used for a melee spell attack

 

No, it would have to specify that it uses up your action, such as saying you can attack with it "if you have not already used your action" or "when cast, you can use your action to..."  or have some clause that you can't have already used your action for something else.  It would also make it have a bizarre casting time of 1 Action AND 1 Bonus Action. 

Spells do what they say, this one says, "When cast, you can make a..."  Your interpretation would be like saying spells that have a casting time of 1 Action and call for an attack roll somehow actually take a second Action to cast them (1 to cast, 1 to attack).

So with this spell, let's say it becomes my turn, can I: 

1)Cast Spectral Weapon as a BA right in someone's grill (as long as it in range, of course)

2)Step into melee with a creature

3)Attack creature with held melee weapon

4)Attack creature with spectral weapon

5)End turn

Because it says casting it is a BA but it never describes using it as a BA. Could it be considered a free action to use it? Just speak the words "attack now"??

And if I cannot do that, can I use Spectral Weapon as a second attack on my next turn (after it has already been cast)??

Can an Oath of conquest paladin "technically" smite with this weapon? It's your weapon under your control... am I overlooking some wording anywhere?

Druid and Cleric seem to get a lot of these spells that have a continuous effect but don't take up your concentration 

The paladin's Divine Smite feature can be used "when you hit a creature with a melee weapon attack". This spell allows you to "make a melee spell attack". In d&d, weapon attacks and spell attacks are two separate things. Aesthetically the line between the two can be blurred sometimes, such as with this spell, but since Spiritual Weapon specifies that you're making a spell attack, it can't benefit from a Divine Smite. Obviously the DM can rule otherwise, but normally that's how it works.

is the at higher levels footnote worded in a confusing way to anyone else? when you cast this spell slot of 3rd level or higher, the damage increases by 1d8 for every two slot level above 2nd, meaning 3rd level cast doesn't increase damage, correct? 2d8 at level 4, 3d8 at level 6, and 4d8 at level 8? is it just worded this way because you can cast as a 3rd level slot if you're out of 2nd level slots, but with no increase in damage output?