Mine is in progress. It's... going to be long. Just a little advance warning.

EDIT: And here it is.
This boss is being made under the assumption that Link starts with three hearts and there were 7 other major bosses prior to this one and the player did not go looking for other heart pieces outside of them. (In other words, this is being made with the assumption that the player has a bare minimum of 10 hearts) This battle also assumes that the player already has a Magic Gauge or will get one sometime directly afterward.
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Name: Kailor Tretton, guardian and holder of the secret knowledge of the Dragonflame’s location
Human form: 5’8” feet tall with a lean build, Kailor is of average height and weight. He has chin-length black hair that is very wiry in texture and sticks in every direction haphazardly. He has ruby-red eyes with slits for pupils, a finely carved nose and a stronger jaw line. He wears a series of black, white and gold robes with a pair of dark brown leather shin-high boots and gloves underneath a set of light steel spaulders, gauntlets, elbow guards, shin-guards and a chest plate. He wields a wicked looking bastard sword in his right hand with magic runes engraved in the blade and dagger in his left.
Half-shift: Kailor sprouts a pair of great, black leathery wings and a long black-scaled, spiked tail. His nails extend into claws that protrude through is gloves and the holes in his gauntlets and two horns grow from his head. He grows about three feet as well.
Dragon form: His dragon is four stories tall and 50 feet long with inky black scales, two great horns, wings and a long tail with spikes running from the top of his head down to the end of his tail.
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The
Dragonflame is a one-handed scepter made mostly from the bones of a long-dead, ancient dragon and topped with a miniature version of a dragon skull with red rubies set in the eyes, its maw usually closed. On the user’s command, the head opens its mouth and releases pure dragon fire from its mouth in a cone that extends out to 30 feet. The fire is creates is hotter than any other, natural or magical, and burns right through most defenses unhindered. It draws the energy used to create the fire from its user. After each burst of flame, the Dragonflame requires a few seconds to cool down before it may be used again. It may also be used as a bludgeoning weapon.
It is a fairly powerful artifact, but never, ever have it out and in sight when around other dragons or else they will attack you immediately.
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Kailor first appears in Hyrule Castle Town in a building and only says a few lines to you. After the defeat of the 7th Story-critical Boss and when the player meets certain criteria, he appears in other areas. The player must have attained the Master Sword by this point, or some stripped-down version of it (Kind of like in Wind Waker). The player must also be in human form when first approaching him (assuming there are sequences like in TP where Link can take on some animal form or such).
Kailor can be found in one of three areas: 1.
Death Mountain: In the morning before noon, Kailor can be found lying on his back in a patch of grass off to the side of the path leading up to the Goron’s domain. In order to wake him up, you can do one of two things: attack him or play whatever instrument you have on you. If you attack him, he will respond in kind with an attack that knocks out 3 hearts; then battle begins immediately. If you play an instrument, he will sit up and you can talk to him.
2.
Hyrule Lake: During the afternoon and early evening, Kailor can be found here either fishing or going through his paces with his weapons. Again, if you attack him, he will strike out, taking out 3 hearts and then the battle begins or you can talk to him. If you approach him in some form other than human, he will ignore you.
3.
Hyrule Field: Kailor can be encountered wandering Hyrule Field at night. He starts near the entrance to Kakariko Village and walks a circuit around the entire area before returning there and flying away. If you approach him in some form other than human, he will ignore you. If you attack him, he will bat you away, damaging 3 hearts, and the battle begins. Approach him in human form and start a conversation. If you have met the game’s criteria, you will have the option to ask for information on the Dragonflame. Otherwise, he’ll say a few words and just move on.
The battle against this boss takes place in phases depending on where you find him:(Also, any monsters that might otherwise interrupt avoid the battle for fear of being killed by Kailor)
Death Mountain:Phase One –If you simply attacked him, Kailor will draw his weapons and begin attacking. In this phase, he is in his human form and follows a fairly simple attack pattern. If you are within clawshot range of him, he will charge at you and attempt to attack you with his sword and will usually do 1 heart of damage. If you wait and defend, he will strike 5 times. The fifth strike is one strong enough to force to slide back a step and damages 3 hearts if you don’t block, but it also leaves him wide open long for you to get an attack in. If you attack before this, he will block or parry the strike. If you jump attack at him, he will side-step it and kick you, damaging half a heart. If you spin attack him, he will simply leap back to evade it.
If you talked to him before the fight, if you get down to a single heart or less, the battle ends and he will tell you to come back and try again when you have gotten stronger. If you attacked him first, he will kill you.
After you manage to hit him 8 times, it will go to phase two.
Phase Two –Kailor will half-shift and maintain an altitude where you can reach him with your sword and will deflect any projectiles you send his way and then spit a fireball out at you that takes away 3 hearts. What you must do is clawshot his tail. When you do, he will start flapping his wings harder and fly up into the air, taking you with him. You will have to follow some simple instructions in order to hang on, but once you get past this, he will land in a rocky outcropping up above the path that cannot otherwise be reached. If you fail to hang on, then you will be dropped to the path below and take falling damage depending on how high you were before you fell (from half a heart, to three hearts) and the battle will be over and you will have to try again later.
When he lands on the outcropping, he will deposit both of his weapons and use his claws and tail to fight. The outcropping will be dotted with grass and old pots you can break for items and a few cuckoos meander in one corner. If you get up close, he will slash at you three times with his claws and then sweep his tail along the ground to trip you up. Each claw attack deals a heart of damage, the tail sweep does no damage, but whether or not he manages to trip you, he will body slam you and deal 4 hearts of damage. If you dodge it, he will be open and you can attack. Every few minutes, he will rear back, taking a deep breath and then spit a fireball at you. Unless you have a magic shield, it cannot be blocked, you must dodge or lose 3 hearts and take a quarter heart of fire damage over each of the next four seconds. If you attempt to jump attack him when his guard is up, he will catch the blade in one clawed hand, laugh at you and punch you back, dealing 3 hearts of damage. If you spin attack, he will jump into the air and smack you in the face with the end of his tail, knocking you prone and dealing 2 hearts of damage.
If you talked to him before the fight, if you get down to a single heart or less, the battle ends and he will tell you to come back and try again when you have gotten stronger and you will have to use one of the cuckoos to get down. If you attacked him first, he will kill you.
After you manage to him 10 times, it will move on to phase three.
Phase Three – Phase three is unique for each area. For Death Mountain, Kailor will rise up over the rocky outcropping and shift into his dragon form, hovering in the air with each flap of his huge wings. He will roar at you and start flapping his wings furiously to create a gust of wind. Either be in front of a large rock or put on the iron boots (assuming the player has them) to keep from getting sent flying off of the edge. Then he will roar again and fly off, swooping around the outcropping. Here, he will launch large fireballs at you that deal 5 hearts of damage, but are fairly slow-moving and easy to avoid.
After a pass, he will land on one of the upper ledges, pan in close with his head and roar at you. In that instant, you need to swing at his head with your sword to hurt him or he will try to spray you with a blast of fire. Getting behind a rock is the only way to avoid it if you don’t attack him in time.
If you talked to him before the fight, if you get down to a single heart or less, the battle ends and he will tell you to come back and try again when you have gotten stronger and you will have to use one of the cuckoos to get down. If you attacked him first, he will kill you.
After you manage to hit him 5 times, the battle will end, he will shift back into human form and then you can get the information you want and a new full Heart piece. Then you will be given a choice: to land the final blow and kill him, or to spare his life. You don’t lose your rewards for winning if you kill him, but if you let him live, he will show up a few more times later on to give you assistance and to teach you something.
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Hyrule Lake:Phase One –Same as in Death Mountain. Hit him 8 times and it moves on to Phase Two.
Phase Two –Kailor will half-shift, roar and start to turn around. In which, you will have a few seconds to clawshot his tail. If you don’t do it in time, you will have to chase after him on foot. If you get him, he will still take off, dragging you along the surface of the lake. You will have to follow simple instructions to keep from losing your grip. If you lose your grip, you will take half a heart damage and will have to chase him the rest of the way by swimming. If you manage to hold on, he will land on the other side of Hyrule Lake and your fight will continue there. Combat after this is identical to Death Mountain. 10 hits to go to Phase Three.
Phase Three –For Hyrule Lake, Kailor will rise into the air and shift into his dragon form. At the start, he will start flap his wings a few times and come diving towards you. If you don’t dodge his head, he will have captured you in his mouth. At which point, if you attacked him first, you will have to follow a few instructions to avoid being devoured and take 2 hearts of falling damage if you succeed. If you don’t, he swallows you and you die. If you talked to him first, then he just chews you up a bit, dealing 5 hearts of damage before spitting you out and dealing another 2 hearts of falling damage, unless you fall into the lake.
After that, he will start circling over head in the air and spit fireballs at you, much like in Death Mountain and you must dodge them or take 5 hearts of damage. If you are in the lake, that damage is reduced to 2 ½ hearts and if you dive underwater, you take no damage. Once he’s made a few passes, he will land on the shore and roar at you, crowding in close with his head. You need to swing at him to damage him and if you are too late, he will spew a lot of fire at you dealing 5 hearts of damage if you can’t evade it or 2 ½ hearts if you’re in the lake or 0 if you dive underwater.
Also, just for Hyrule Lake, if you can clawshot his tail while he’s flying around, put on the iron boots, and manage to force him to fall into the lake, he will be weakened and only deal half-damage on all attacks after that. If he just lands on the ground, he’ll be stunned long enough for you to get in a good attack.
If you talked to him before the fight, if you get down to a single heart or less, the battle ends and he will tell you to come back and try again when you have gotten stronger and you will have to use one of the cuckoos to get down. If you attacked him first, he will kill you.
After you manage to hit him 5 times, the battle will end, he will shift back into human form and then you can get the information you want and a new full Heart piece. Then you will be given a choice: to land the final blow and kill him, or to spare his life. You don’t lose your rewards for winning if you kill him, but if you let him live, he will show up a few more times later on to give you assistance and to teach you something.
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Hyrule Field:Phase One –Same as in Death Mountain. Hit him 8 times and it moves on to Phase Two.
Phase Two –Similar to Hyrule Lake, except he drags you across the field instead and lands on the other end of the fields. 10 hits to get to Phase Three
Phase Three-For Hyrule Fields, after Kailor shifts into his dragon form, he will do the same charge as Hyrule Lake and try to catch you in his mouth. Same basic premise, except there’s no way of avoiding falling damage should he catch you. Combat is identical as Hyrule Lake, minus the ability to drag him into water. Kailor also will not use his breath weapon while on Hyrule Fields, as he does not wish to set the whole field ablaze. Instead, he will do more diving attacks. Also, occasionally, he will land, try to tail sweep you and then body slam you. If he succeeds on said body slam, he does 8 ½ hearts of damage. If he misses, he is stunned for a second and you can strike him.
If you talked to him before the fight, if you get down to a single heart or less, the battle ends and he will tell you to come back and try again when you have gotten stronger and you will have to use one of the cuckoos to get down. If you attacked him first, he will kill you.
After you manage to hit him 5 times, the battle will end, he will shift back into human form and then you can get the information you want and a new full Heart piece. Then you will be given a choice: to land the final blow and kill him, or to spare his life. You don’t lose your rewards for winning if you kill him, but if you let him live, he will show up a few more times later on to give you assistance and to teach you something.