Dying Flame v4

The explosion of disease infected him; the disease festered inside his body. It nearly killed him before he made a desperate attempt, setting himself on fire to burn out the disease. It didn't rid the disease out of him fully, and still threatens his life, but no longer constantly getting worse. Pyro's body still holds the disease that is slowly killing him; only his unnaturally high body temperature and constant exposure to fire hold the disease at bay or even regresses it. 

Creeping Disease. Whenever Pyro is exposed to cold temperatures for an extended period of time, each hour he must make an increasing DC 10 constituion saving throw, increasing by one for each time he succeeds, and resets when he takes a long rest, making these saves every hour exposed to the cold temperature. On a failure, he takes 1d6 poison damage, and the poison advances one point further into his body. Taking a short rest resets the poison advancing points by one, and a long rest does it by two. If the poison advance reaches 5, he receives a permanent lingering injury of appropriate choice by the DM, and can't regress past 5 using long rests or short rests. Only regressing if the DM allows an effect to regress. After which, gaining a temporary lingering injury after every three poison counters. If the poison advance reaches 12, he falls to death saving throws. And the poison can't regress back one more than the previous (Ex. first time he falls to death savings throws from this it can't regress past 6 using long rests or short rests, then 7 the next time, then 8, and so on and so forth.) 

The DC to stabilize Pyro with a medicine check increases to 18.

Whenever Pyro takes cold damage, he must make a Constitution saving throw equal to half the damage or 10, whichever is higher. On a failure, the poison advances by 1, and he takes 1d6 poison damage.

If Pyro receives healing or gets stabilised while in death saving throws due to Creeping Disease, the poison regresses two, and his maximum hitpoints are reduced by 10 until he takes a long rest. If this causes his hitpoints to go below zero, he dies.

Inferno. When Pyro uses Flames of Phlegethos, he can choose to enhance the fire by expending a first-level slot or higher. Doing so turns the fire blue and makes it more deadly. Whenever a creature takes fire damage from Flames of Phlegethos it takes an additional 1d4 fire damage. While Flames of Phlegethos's effect is active, Pyro doesn't have to make any saving throws when exposed to cold from his Creeping Disease feature.