Need help with throne room encounter! (5e)

[PeaceThroughWar]

New to 5E DM looking for help converting an Ecstacy Feeder from Pathfinder (Dont have enough posts to link) to a CR 6 monster for my campaign and encounter design help.

The party (3 -5 pcs depending on who can make it) will most likely be level 3 or 4. The encounter will take place in a throne room with neutral/friendly king, queen, probably several retainers (all non combat) and maybe 4 - 6 guards (probably level 1 fighter equivalents).

Background: Horror campaign using sanity scores. Homebrewed Elder Evil focused around stealing... for lack of a better word.. essence.. from the world (Joy, happiness, knowledge.. the intangible stuff) As the EE gains power, abberations with its focus in mind become more prevalant. As the PCs are being introduced to the ruling royal family, an "envoy" enters the room under a similar purpose as the PCs.. only to reveal themselves and attack. I have never converted monsters before and was hoping someone who has mastered the art of encounter design could help.

[Hexblade]

Wow. This seems really interesting. Too bad I don't know 5e...

[PeaceThroughWar]

Yea Im homebrewing the entire campaign from scratch... its a lot of work but a lot of fun too.