If you read My Haunted House of Horrors (MHHH from now on) you will find a lot of red, resesment, and supernatural melding with normal world.
Now I wish to present you to Grimm Troupe from Hollow Knight. They are servants of Nightmare Hearth who travel trough time and space to colect the red nightmare flames to strenghten the Hearth. They are eternal undiing.
What I propose is that you can easily make Chen Ge (MC of MHHH) stumble upon them and start the 'Ritual' which will force him to colect the flames for them and slowly feed the Grimm Child to become the next Troupe master.
Depending on how you write this it can be realy short. To Extreamly lenghtly. MHHH has something caled Trial Missions. Each flame could be one mission because they are located in variouse locations trought the 'Empire'
You can also just incoporate the Grimm Troupe into the original story to make it AU. Flames being just another thing to look out for.