None but (they/them) is/are [closed]

Can someone tell me how to make a sentence with "none but"?

None but they/them is/are responsible for it.

The difference between "they" and "them" is that "they" is referring to the subject, while "them" is referring to the object. An example taken from here:

They is used to refer to the subject of a clause. In other words, it usually represents the ‘doers’ of the action described by the verb, and usually refers back to two or more people or things that were mentioned earlier:

The children were kind. They gave me a present.

And

Them is used to refer to the object of a clause. In other words, it usually represents the group of people or things that have ‘experienced’ the action described by the verb, and refers back to two or more people or things that were mentioned earlier:

I’ve bought some apples. I’ll put them on the table.

The people who are responsible in your sentence is the subject.

They (subject) are responsible for it (object).

It should be "are" instead of "is" because "they" is a plural.

None but means only. [formal] (Collins Dictionary)

None but God will ever know what I suffered.
He whispered so softly that none but Julie heard him.

"They" is subject pronoun and "them" is object pronoun.
You should use "they" in the sentence as the subject.

Verb should agree with subject so "are" is used here.

None but they are responsible for it.