SubtraCo.

The Subtraction Company, or SubtraCo. for short, is a rival group of adventurers to the PC's party introduced when the players first reach Travlers Town.
As the party defends the town from the Skeleton Bandito invasion, they will be assisted by 4 adventurers of great skill:

• Cecil Lavernius - a female Water Genasi Pact-of-the-Blade Warlock - Party Leader
• Emera Rose - a female Tiefling Sorceror with pink skin and horns
• Wyatt Pyth - a male Yuan-Ti Cleric with yellow scales
• Kay Vantas - a female Dark Elf Monk

Once the battle is subsided, Presto Gravey will formally introduce the two parties. SubtraCo is Presto's other team of adventurers. Just like the PC's party, he offered them the chance to take up missions and use their unique strength to help protect Kwilterra.
If asked about their missions they and Presto will remark that there is a general air of secrecy to their work. This way, should anyone be captured, the Phantom King's forces can never get the full scope of their efforts. Even Presto doesn't know everything.

Encounter

Kay will challenge the main party to a friendly team sparring match (after they've all rested) to further judge their strength in combat. If denied, she will be disappointed but maintain a cheery attitude.
If engaged, SubtraCo will fight the party with non-lethal strikes.

Growth

A mechanic of SubtraCo as rivals is that as the party goes on missions, so do they, and they level up relatively.
When the party first encounters them, SubtraCo will have the level advantage (starting the campaign at lvl 5 and clearing the 4 hostile towers of Attitude City puts the party at lvl 9 when they reach Trav'lers Town).
As the party continues to grow, they begin to close the gap with SubtraCo, who will occasionally challenge them to sparring matches to gauge both parties' advancement. Starting at level 15, the main party will catch-up to SubtraCo and begin to outpace them.

• For PC levels 9-11, CMYK will be equivalent to level 11
• For PC levels 12-14, CMYK will be equivalent to level 14
• For PC levels 15-17, CMYK will be equivalent to level 15
• For PC levels 18-20, CMYK will be equivalent to level 18

DM Note: lvl 15 is also the minimum for completing the story as that's the lowest the party can be after doing only the main quests before challenging the Phantom King.

PC Party Levels	CMYK Levels
9	11
10	11
11	11
12	14
13	14
14	14
15	15
16	15
17	15
18	18
19	18
20	18

End Game

SubtraCo can be asked by the party to assist with the final raid on Casper Castle, the Phantom King's floating Fortress, but without an especially difficult persuasion check (DC 19), they will decline to stay on the ground and protect the remaining citizens of Kwilterra.
If the party successfully recruits SubtraCo to their final mission, the four NPCs will aid the party up until one of the gatekeeper bosses (depending on which entrance the group decides to use). They will then stay behind and fight the boss, allowing the party to move on without fighting that encounter.
The party can also chose to use them as a distraction, or think of some other way to utilize their additional allies, but one way or another, the DM should try to remove them before the Party confronts the Phantom King as he will focus on taking them out first to use as additional HP banks through his soul drain ability.
The higher leveled the PCs, the more HP SubtraCo will have for him to steal.
In any other scenario besides bringing them into the final battle, SubtraCo will survive the end game and continue their adventures as a group post-credits roll.
Assuming the Phantom King doesn't win and ravage the Prime Material plane of life to fuel his ascension.

Type

Special Forces