Hi guys. I'm planning a cursed city campaign, and there's going to be a sewer section, wherein the players will do battle against hordes of abnormally huge rats and swarms of regular rats. At the climax of the encounter, my players will do battle with the Rat King; a terribly evolved rat hellbent on the conquest of the surface world in the name of rat-kind.

I've drawn up some stats for him, is he appropriately powered? What CR would he be? He started life as a buffed Gnoll fang of Yeenoghu. To be fought by five level four characters.

Skrul the Rat King
Medium Fiend
AC 14
Hit points 80
Speed 30ft, 20ft climb

Str 17, Dex 15, Con 15, Int 10, Wis 11, Cha 16

Saving throws - con +4, wis +2, cha +3
Senses - Darkvision 60ft, passive perception 10
Languages - Common, Abyssal


Vermintide; As an action, spawn 1d4 swarms of rats within 5ft of Skrul. Swarms spawned in this way have 12hp rather than 24.

Multiattack; Skrul makes three attacks. One bite attack and two claw attacks.

Bite; +5 to hit, reach 5ft, one target, 1d6+3 piercing

Claws; +5 to hit, reach 5ft, one target, 1d8+3 slashing

Spellcasting (charisma as casting stat)
At will - poison Spray
2 casts/long rest - ray of sickness

The general idea is that he will spawn alone and then create his own reinforcements as the fight progresses.