And they have plotted a tremendous plot.
"They schemed terrible schemes.
they have devised a mighty plot
And they have planned a mighty plan
And they cheated/deceived , a great/magnified cheatery/deceit
Their chiefs have already devised a vicious plan against them
“And they carried out a very sinister scheme.”
`And they planned a mighty plan.
And they kept perpetrating huge frauds (to keep the common man in error)
They have devised a mighty and heinous plan (against the Prophet)
"And they have plotted a mighty plot