Is there anything in 3.5e lore about a mortal ingesting the body parts or blood of a Deity? Or the effects that something like that would have on a PC?
For context, I play a Dragonborn Bard in possession of a jar of "dragon ichor" and some eye juice from the eye I stole from the corpse of Tiamat, and I've been considering mixing the two and drinking them. Obviously, this precise sequence of events has never occurred before in D&D history, but has anything remotely similar happened in D&D lore? Any situation in which a mortal ingested a portion of a deity.
Best Answer
Absorbing a God's Power? Yes
We've got some examples of this, most pertinently the Birthright Campaign Setting in 2E, which received a bit of updating in Dragon Magazine. We've also got the Lich-Queen Vlaakith, who works to slowly absorb the power of the god-corpse known as The One in the Void. In both of these cases, there is no ingestion, and the process involved provides only a fraction of the power of the divine. Let's look at what we could do with this.
Mythological Antecedents
The most obvious one that comes to mind involves a popular religion where bread and wine are transubstantiated on ingestion into the flesh and blood of a divine avatar. Not only is this super safe, it's also a way of reinforcing a covenant between the human doing the ingesting and the deity doing the transubstantiating. Mind you, there are a looooot of filters here, most importantly that the physical material in question is not actually any body part of a divinity. That property is, under many readings, considered to be conferred onto baser materials in a spiritual sense.
More interesting to me is the mythological pattern of creatures or divine beings whose blood is incredibly dangerous - from the corrosive blood of Fafnir to the toxic blood of the Hydra and the basilisk to the regenerative blood of the Asura Raktavija. The Greek gods were known to share this property - the ethereal fluid that flowed in their veins, ichor, was said to bring guaranteed death to any mortal who touched it.
Back to Birthright
Now, in the Birthright campaign setting, the deaths of the old gods caused their divine essence to explode outward and get absorbed into the mortal armies who surrounded them on that fateful day. Fractions of divine power fused with the very blood of these mortals, creating bloodlines who can pass down some sliver of their own power to their descendants.
More interestingly, and closer to your question but still a bit away from it, is that those possessed of such a blood heritage could absorb strength from others so empowered in a Highlander-esque manner - by dealing out a deathblow to a rival with a stab through the heart, the victor would take a piece of the loser's blood strengh, potentially gaining new powers in the act of having done so. Even under optimal circumstances, the full power of the defeated mortal would never transfer over (best case is about half).
Part of this fun with godly essence is that those possessed of blood powers could potentially link with the land, creating a pseudo-divine realm for themselves tied to their will, a process called Regency.
But the Cosmic Cocktail? The Divine Draught?
...has strictly speaking not happened in D&D lore that I can tell, hence the inferences above. We can see that the mythology that inspires D&D has some things to say about ingesting god juice being a very poor plan, as well as that D&D has some notions about godly powers being transferred in some lesser capacity through blood (though not in a directly vampiric way).
My Recommendation
Based on all of the above, I would like to make an important posit: god-stuff is not something to toy with lightly, and directly ingesting it is likely a fantastically excellent idea if you're into suicide and an abominably stupid idea if you're not. However, we can look at the Birthright principles of investiture (a way of choosing to pass on your blood powers to another through a ritual) and usurpation (performing a specific stabby act on another's heart to get blood power) and notice that there's something of a process to go through that makes it all work. Similarly, if we look back at the Sacrament of Communion, nobody's wandering up to a tabernacle and snacking on Eucharist like potato chips - it gets brought to an altar, blessed by a priest, prayed over, and presented in a holy sacrament. It's a process - there is a ritual to be observed around the ingestion of the essence of the divine.
Therefore, what I would suggest - and yes, this would involve working with the DM, as needs must - is that absent appropriate preparations, ingesting a Tiamat Tonic should very very rapidly kill you deader than dead. However, through research and preparation, your character should be able to discover or otherwise devise a rite that can be followed, likely involving the inclusion of other magical ingredients to stabilize the mixture and prevent it from immediately murdering you, which upon imbibing should confer some amount of unique power. God-level? No, the godhead is not so easily transferred as that. Superhuman? Absolutely.
Given that it's Tiamat, you might find such a cocktail gives you draconic eyes and turns your skin scaly. Perhaps it imparts the half-dragon template and some unique spell-like or supernatural abilities. It should be expensive and dangerous - that's what makes such endeavours feel earned, worthwhile, and of course fair to your fellow players at the table. Some other notions include:
Your DM is of course welcome to say "no," but at the very least you have a basis for exploring this idea with them. The remnants of a god are basically relics in a literal sense (see definition 3) and there are good thematic, mythological, and lore-adjacent reasons to imagine that ingesting such a thing could have a powerful effect.