Dracoconvergence

True Dragons are magical by nature, and even the dragons themselves cannot explain every ability they possess in a technical manner understood by mortal scholars. One such mystery is Dracoconvergence, the ability of a dragon to take over a mortal body by transferring their soul into it. This renders the draconic body dead, and as such was not usually done save for the moment of death itself. The dragonsoul has access to all the memories and abilities of their host, as well as some latent draconic abilities, such as the ability to climb stone walls in a copper dragon. However, the dragonsoul loses access to it's breath weapon, spellcasting, flight, and ability to polymorph. Dracoconvergence is considered cowardly to most dragons, or otherwise humiliating, though it was used very frequently during the Dragon War, when dying dragons would steal the body of soldiers in order to serve as spies for dragonkind, or otherwise disrupt mortal armies. The dragonsoul is capable of pretending to be the host perfectly, with even close friends and family unable to tell the difference if the dragonsoul is committed to the act.
The mortal soul of the possessed body is not expelled, it remains inside and is capable of speaking with the dragonsoul mentally, but is mostly helpless. Hosts with strong enough willpower can take control of their bodies for short amounts of time, though records indicate that some extraordinary individuals were capable of fully suppressing the dragonsoul forever. The mortal soul also tends to influence the personality of the dragonsoul over time, the longer a dragonsoul possesses a mortal the more obvious these changes become. In situations of extreme danger to the host body, the mortal soul sometimes takes control temporarily as a panicked defensive measure, preventing the dragonsoul from doing something potentially lethal.
The sudden exposure to mortal pain, feelings, and sensations occasionally drive the dragonsoul to madness. These dragonsouls will often lose themselves in sensation, unable to stop themselves from acts of extreme hedonism or self-harm, as the ability to taste or feel pain is entirely unknown to dragons as mortals understand it. Even non-mad dragonsouls have a particular weakness for alcohol, as it cannot affect their draconic forms. They also tend to prefer their meat undercooked, but not to a degree that would be dangerous to the host body. Other behaviors can become addicting to dragonsouls possessing mortal hosts, such as running to the point of exhaustion or sleeping, as dragons cannot dream otherwise. 
A dragon-possessed mortal body can still be the target of Dracoconvergence. In these rare cases, the two dragonsouls function as a hivemind, reaching all decisions through consensus faster than mortal thought can follow. One known instance of five dragonsouls in a single mortal host did create a situation in which the host would often freeze for hours as the different dragonsouls argued over which course of action to take, most notably, what to eat that day.