Comparative Anatomy

"To heal is to understand life. To poison is to understand how it ends. True mastery lies in knowing that these are one and the same."

Years spent dissecting beasts, cataloguing anatomy, and painstakingly comparing the physiology of countless creatures have led your research to a conclusion few would dare entertain. Nature does not create perfect organisms, rather it scatters perfection across thousands of different lives. Every creature possesses some adaptation refined through generations of survival, a gift unique to its own flesh.

Most anatomists have sought only to understand these adaptations, while you’ve sought to inherit them. Through years of experimentation you have discovered that living tissue, when properly prepared, can survive transplantation between entirely different species. By replacing parts of your own body with those of another creature, your physiology slowly becomes a mosaic of nature's finest designs. Every successful graft leaves less of the body you were born with, replacing it instead with organs that have already proven themselves against the countless trials of evolution.

Since nature has already solved every problem, you have simply learned how to steal the answers.

You gain the ability to permanently graft up to three biological organs onto your own body. Harvesting an organ requires undertaking a targeted quest in which the desired creature is defeated. Only organs harvested from Beast or Monstrosity creatures may be grafted in this way, and organs from homebrew creatures, as well as named creatures, cannot be harvested.

The harvested organ must also remain in a sufficiently intact condition to survive transplantation. A creature that has got the objective organ destroyed, damaged, or otherwise made inept cannot be harvested. This determination is made by the DM.

Within 10 minutes of the creature's death, you may use an Action to begin the transplantation, first removing the corresponding organ from your own body before attempting to graft the harvested replacement. Make a Nature check with a DC equal to 10 + harvested creatures constitution score.

Regardless of the outcome, your own organ is removed before the transplantation is attempted.

On a successful check, the graft successfully bonds to your body.

On a failed check, the graft is rejected immediately, the harvested organ is destroyed beyond recovery, and the corpse can no longer be harvested.

The loss of your own organ carries severe physiological consequences that cannot be restored naturally and remain until the missing organ is replaced through a successful graft or 4 irl days of continued cast of the Regenerate spell.

There are three categories of organs that may be grafted. You may only possess one graft of each category at a time. Grafting a new organ of a category you already possess permanently destroys the previous graft.

Eyes: You gain one of the grafted creature's natural senses, up to the range it possessed. This cannot grant supernatural senses such as Soulsight or other non-natural forms of perception.

If your transplantation fails, you become permanently blind. You cannot make ability checks that rely on sight, and you have disadvantage on ranged attack rolls made beyond a weapon's normal range. These effects cannot be negated except by having the Regenerate spell cast on you for 4 irl days.

Internals: You gain one passive feature possessed by the grafted creature. This cannot include Spellcasting, class features, regeneration, or other features determined by the DM to be inappropriate.

If your transplantation fails, your physiology struggles to sustain itself. Your hit point maximum is halved, you have disadvantage on all Constitution saving throws, and you cannot remove levels of Exhaustion by any means other than replacing the missing organ or through 4 irl days of regenerate spell being cast. 

skin: You gain one damage resistance, damage immunity, or condition immunity naturally possessed by the grafted creature.

If your transplantation fails, the loss of your skin leaves your body dangerously exposed. You gain vulnerability to slashing damage and cannot wear medium or heavy armour. These effects persist until the missing tissue is replaced or restored through the Regenerate spell.

Adaptive Activation

Although your grafted organs permanently replace the anatomy they were transplanted to substitute, the extraordinary adaptations they possess remain dormant until deliberately stimulated. Through careful control over your own physiology, you may temporarily force these foreign organs to express the traits they evolved to possess. Each successfully grafted organ possesses 3 activation charges. As a bonus action, you may expend one charge from a graft to activate its extraordinary adaptation for 10 minutes, gaining the benefits granted by that graft.

Once all Activation Charges of a graft have been expended, it becomes dormant and cannot be activated again until you complete a long rest, at which point all expended charges are restored.

Whenever you are reduced to 0 hit points, all currently active grafts immediately become dormant, regardless of any effect that would activate upon your hit points becoming 0. Their remaining duration immediately ends, though they retain any unspent Activation Charges.

Whenever you take damage equal to or greater than 5 times your level from a single source, you must succeed on a Constitution saving throw against your own graft DC. On a failed save, all currently active grafts immediately become dormant as the sudden trauma forces your body to divert its resources toward preserving the transplanted tissue.

Biological Rejection

Even a successful transplantation is not guaranteed to last. Foreign tissue is constantly attacked by your body's immune system in an attempt to reject it.

At the end of each Long Rest, make a Constitution saving throw for each graft you currently possess. The DC equals 20, increasing by 5 for each graft currently attached to your body (maximum DC35).

On a failed saving throw, the graft you're carrying gains one rejection stack.

After 3 rejection stacks, the graft becomes dormant, no longer providing its extraordinary benefits but continuing to function as a normal replacement organ until the stacks are removed.

After 5 rejection stacks, the graft is completely rejected by your body, immediately ceasing to function and becoming inert, providing no benefits until replaced through another successful transplantation.

To remove a Rejection Stack once obtained, you'll need 3 castings of greater restoration, each cast on separate days and targeted specifically at the affected graft.

A graft rendered dormant through Biological Rejection cannot be activated through Adaptive activation until its Rejection Stacks have been sufficiently removed.

Should your body ever be completely destroyed and later restored through means of a true ressurection, wish or any other effect that recreates an entirely new body, your grafted organs are not recreated alongside you. Instead, your body is restored with its original natural organs intact, permanently losing any transplanted grafts previously attached to your body.

Lastly, creatures whose biology you have incorporated into your own are strangely familiar to you. You gain a bonus equal to your proficiency bonus on ability checks and saving throws made against creatures of the same species as any of your current grafts.