The Arsenal Falcon Engine (feat. Kashtira)

Raidraptor - Arsenal Falcon Rank 7 DARK Winged Beast 2 Level 7 monsters Once per turn: You can detach 1 material from this card; Special Summon 1 Level 4 Winged Beast monster from your Deck. If this card has any number of "Raidraptor" monsters as material, it can attack up to that many times during each Battle Phase. If this card is sent to the GY while it has a "Raidraptor" monster as material: You can Special Summon 1 "Raidraptor" Xyz Monster from your Extra Deck except "Raidraptor - Arsenal Falcon", and if you do, attach this card from your GY to it as material. ATK 2500 | DEF 2000

Guess who is a Level 4 Winged Beast?

This card can Summon Fire King Courtier Ulcanix directly from the Deck, jumpstarting our combo. As such, any engine capable of making a generic Rank 7 Xyz Monster can find its place in a Fire King deck.

One of the most popular choices for that is the Kashtira cards - almost generic staples that besides swarming the field and being Level 7, can add value to your deck through their disruptive effects. Thus, you might see a tiny Kashtira core in some Fire King lists, like this one:

3x Kashtira Fenrir 1x Kashtira Riseheart 1x Raidraptor - Arsenal Falcon 0-3x Pressured Planet Wraitsoth

Fenrir + any FIRE monster is full Fire King combo:

• Special Summon Kashtira Fenrir.
• Activate Fenrir's effect to add Kashtira Riseheart from Deck to hand.
• Normal Summon Kashtira Riseheart.
  • Why not Special Summon? Because Riseheart locks you into Xyz Monsters for the rest of the turn. Normal Summoning avoids this.
• Xyz Summon Raidraptor - Arsenal Falcon using Fenrir and Riseheart.
• Use Arsenal Falcon's effect to Special Summon Fire King Courtier Ulcanix from Deck.
• Proceed with the 2-card, Ulcanix + FIRE combo.

As you can see, this can be a simple way to increase the consistency of your Fire King deck, as long as you have a playset of Fenrirs available.

A note on Fire King Kashtira

Seeing this combo might prompt you to explore a larger Kashtira engine, adding cards like Kashtira Unicorn, Kashtira Birth or Kashtiratheosis. While these cards are good cards, it's easy to get lost in the sauce and end up with a deck that either plays the Kashtira half, or the Fire King half, instead of both archetypes working in unison towards a single goal.

Whether this is preferable to you depends on your deckbuilding goals.