Dragon Lycanthropy (5e Variant Rule)
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DM Note: This ruleset contains use of Scale Sickness as a base. If you wish to use it, you should look over that page as well, though use of the disease aspects of Dragon Lycanthropy is entirely optional. In addition, it also makes use of the variant rule in the D&D Monster Manual as a base for how Lycanthropy generally works for player characters, and thus you should be familiar with that rule on pg. 207 before continuing.
Dragon Lycanthropy is arguably the most volatile and dangerous of the types of Lycanthropy, at least partially because of its rarity. Then again, its rarity might be contributed to the clear fact that for most it's deadly to contract. A Humanoid that isn't a Dragonborn or Half-Dragon and the like (They are immune due to their already draconic nature) hit by an attack that carries the curse of Dragon Lycanthropy must make a Constitution saving throw (DC 8 + the Lycanthrope's proficiency bonus + the Lycanthrope's Constitution Modifier, the DC cannot be lower than 20 but can exceed it) or be cursed. Anyone exposed to Scale Sickness that isn't a direct attack must also make a Constitution saving throw of DC 20 or be infected. If using Scale Sickness, a character only becomes a Dragon Lycanthrope instead of a creature inflicted with Scale Sickness if the character has a Constitution score of 19 or higher. A Sorcerer with the Draconic Bloodline archetype will always become a Dragon Lycanthrope, regardless of their Constitution score, and will never get Scale Sickness. If the character embraces the curse, his or her alignment becomes the one defined for the Lycanthrope (Which is directly associated with the dragon type, chaotic evil for red, lawful good for gold, etc.). The DM is free to decide that a change in alignment places the character under DM control until the curse is removed. A Sorcerer with the Draconic Bloodline archetype can embrace the curse but retains their alignment even if they do so.
The following information tells the player what they've won by not falling incredulously ill or dying to the attacker. The character gains a Strength score of 22 if their Strength score isn't already higher, and a +3 bonus to AC while in hybrid form (From Natural Armor). The character grows a coat of scales and turns into their hybrid form, and cannot turn back into their normal form while they have the curse, except through magical means such as alter self. A Sorcerer with the Draconic Bloodline archetype can turn back into their normal form as an action, but its statistics, other than its AC, are the same in each form. They gain natural weapons they can use in hybrid form or dragon form, giving them a Bite attack (1D8 in Hybrid form and 2D10 in Dragon form) and a Claw attack (1D4 in Hybrid form and 2D6 in Dragon form), which are both Melee Weapon Attacks that use their Strength. A Dragon Lycanthrope can choose not to infect creatures it attacks with Dragon Lycanthropy.
They also gain a breath weapon, which is equivalent to a Dragonborn's breath weapon in hybrid form, or an Adult Dragon's breath weapon in dragon form. For either form, they roll a D6 at the start of their turn while the breath weapon is expended, and recover their breath weapon if they roll a 5 or 6. Their hybrid form also gains wings that allow a flight speed equal to their walking speed, but these wings only grow 7 days after being infected, unless if the character is a Sorcerer with the Draconic Bloodline archetype, in which case they form immediately.
A Dragon Lycanthrope can exist as any type of dragon, and gains the appropriate damage immunities. Fire for red and gold dragons, cold for white and silver dragons, etc. Additionally, the creature gains a Shapechanger ability, which allows it to polymorph into an Adult Dragon of its type or back into its hybrid form (Or humanoid form if a Sorcerer with the Draconic Bloodline archetype chooses to) as an action. Its statistics, other than size, hit points, speed and AC, are the same in each form. It assumes the hit points of the adult dragon form, but as a conversion of their normal hit points, and vice versa. In other words, if the character is at 50% of its hit points when it changes into its dragon form, its dragon form has 50% of its hit points as well (Rounded down). A Dragon Lycanthrope that falls to 0 hit points in its dragon form falls unconscious and reverts to its hybrid form. A creature that doesn't embrace the curse turns to an adult dragon on a full moon, and only turns back the next and can't control this form. Additionally, such a creature loses consciousness during this timeframe, and the DM takes control of the character as an NPC adult dragon of its normal alignment. A creature that has embraced the curse can use the Legendary Actions of the Adult Dragon, but are limited, as they only regain 1 spent legendary actions use at the start of each turn.
DM Note: The parts of the ruleset dealing with Sorcerers also apply to other classes and races that have dragon archetypes besides Dragonborn (Or any equivalent to Dragonborn). If using a homebrew class or race that has a dragon archetype that doesn't inherently make the character naturally a dragon-like creature, they gain the benefits that a Sorcerer with the Draconic Bloodline would.