[Grammar] Do I really need a comma after which?

[Ulysses]

Is a comma in the sentence before 'which' here necessary? I can't tell but it seems vague to me:

Crown exercises power through advice of ministers, which is really a command.

 

[tedtmc]

not a teacher

Should there be a 'the' before 'crown' and 'advice'?
I'd say the sentence sounds better with the comma.

 

[Raymott]

Is a comma in the sentence before 'which' here necessary? I can't tell but it seems vague to me:

Crown exercises power through advice of ministers, which is really a command.

Yes, you must use a comma here.
It's a non-restrictive 'which' clause. That is, it adds to the sentence rather than restricting "advice of ministers" to only that advice which is a command.
The meaning is that all 'advice of minsters' is really a command.
Without the comma, it means that only some 'advice of ministers' are really commands, and it is those that exercise crown power.