Worldbuilding great purge

I need a way of legitimizing a purge of all the nobles aside from the royal family. The idea is that the country needs to make a transfer from a feudal empire to a more administratively run empire. The emperor has to initiate the pruge, and will obviously need to gain the favor of the lower citizens and the merchant class in order to maintain power. The nobles will have already taken some minor losses to their power before this point, but what legitimization and process would an emperor who does not want to share power with vassals?

I'd love to answer, but first can you give us the gist of the nation's religion, and a quick rundown on the ruling dynasty's claim to the throne?

A religious purge. Brand them all as heretics and have the clergy create strong ties with the Emperor, in fact have the Emperor take on both the role of Emperor and NotPope.

Ivan the terrible in Russia did it, he took out all the boyars and centralized power entirely under his family

Seize the property of the nobles. Distribute estates and lands to the pasantry.

With no rent on lands or serfs to buoy their coffers, the nobles will quick lose money and authority to do anything.

Just started running something similar in a new campaign. Not only were all the nobles killed (except for one of the player characters), but so was most of the royalty, sans the crown prince, who is now the boy king.

Basically, all the neighbouring nations came together to wage an overwhelming war on a singular but wealthy nation in an attempt to balance trade negations and instate their own foreign policies as right of conquest. The nation in question controlled most of the trade routes between each kingdom, since they basically have to pass through their lands in order to reach any other kingdom, and had a monopoly on the salt mines on its northern shore, never mind being the only coastal nation on their part of the continent.

They let the young prince live so that they could effectively use him as a puppet authority. Otherwise, they destroyed or burned down every fief and estate on their way to the capital, which they also burned down for the most part.

The campaign is set a few years later. The player characters are all members of the king's privy council, with most of them sent by the kingdom's neighbouring "allies".

Deliberately go to war, lose, create a famine from lost resources, travel the countryside and rally the peasants by declaring you had no part in this and it's the nobles fault. Establish a force of middle-men to lead the peasants in terrorizing the nobility.

Cause a disaster (or act on a random one) and cockblock any nobility who try to do anything about it. Use your centralized power to neutralize the disaster yourself.

Use propaganda to get the peasants hating the nobles. Freeze their assets, don't accept their money, take their titles and redistribute their land.

Religion:
A monotheistic religion that worships an enigmatic creator deity. The priests are firmly under the thumb of the emperor or their local lords. There is no central authority of the religion, but it the sect that this nation observes adheres to strict observances of saying prayers and attending services. There are two other sects of the religion that are practiced in other nations that are on and off allies of the empire.

Claim:
The current royal family has been in power for about 210 years making them rather entrenched. They specifically avoid marrying any of the lordly class and either mary outside of the country for political reasons or just marries the middle class of commoners who show promise in fields of learning. The emperor who will be responsible is in his 30s but has only ruled for a few years. He is the second son (third child) of the previous emperor who strongarmed/backstabbed his way into being emperor (as it is custom). The inheritance system is designed to make the toughest and nastiest child of the previous emperor inherit, but the heir is chosen before the emperor is dead to try and avoid succession crisis.

Other notes:
The citizens of the empire are characterized by being stern, stubborn, and focus on community over personal gain. There is a dominant culture/ethnic group that has slowly been mixing/overpowering conquered populations (much like romanized regions of Europe).

>The citizens of the empire are characterized by being stern, stubborn, and focus on community over personal gain.
A purge could easily be justified as being over the corrupt (either real or manufactured) nature of the nobles. Since the citizens seem to be a least mildly nationalistic/loyal to the government have the nobility played up as a conniving "other" that is divorced from the "true" government controlled by the royalty.