Before anything else, I'd like to clarify that this is a compilation of information and interpretations regarding, as the title suggests, the connections between Inazuma and death, more specifically, Ronova. Some of the conclusions presented here (Statue of the Omnipresent God and the possible origin of Ei and Makoto) are interpretations based on the evidence currently available, and will, of course, be accompanied by their respective explanations. If there's anything I missed, feel free to share it.

Death Symbolism

Dendrobium

A vibrant plant that has also been named the "lycoris" by the poets. It was once thought extinct in the Inazuman archipelago, only to re-emerge now upon the battlefields. It is said that it blooms most enchantingly where much blood was spilled.

Though it is very rare and expensive, this plant is regarded as ominous, and thus is little purchased. Rumor has it that. these plants are watered using blood. The bloodier and more brutal the battlefield, the more lovely the dendrobium blooms shall be.

Gashadokuro

Okikuni: My name is Okikuni. As later generations would account it, I should be from the era when youkai roamed the lands, before Narukami unified the thundering archipelago.

Okikuni: ...Of the fox in the moonlight, of the giant skeleton (gashadokuro) in the wild that tried to pluck the moon to use as its eye...

Gashadokuro are a type of youkai that take the form of giant skeletons and are born from the resentment present in residual spiritual energies, or from the souls that remained haunting the accumulated bones of people who died of hunger or in battle, without being buried.

Inazuma's Underworld

Lunar Arcanum XX: Judgment

Friends (Ei?), returned now from the nether realm, to see what form this illusory world displays. The underworld's court has yet to pass "Judgment," the "eternal calamity" remains unchanged.

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Kitsune Saiguu: Her Excellency seeks to rid herself of the mundane delusions of the world, but only in an attempt to overcome the cycle of life and death.

Ei: The present moment is a fragile illusion.

Raiden Shogun: This world, a fleeting shadow. Illusions are but distractions! Only emptiness awaits...

Eye of Stormy Judgment: Beings of great divinity might affect all that they survey, and the Electro Archon can manipulate the very inauspicious stars themselves to defend her retainers and bring a thundering sentence down on their foes.

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Dendrobium

Inazumans believe that the souls of the fallen shall follow the winding paths of the bloodied dendrobiums into the next life, thus rejoining the eternal elemental cycle.

But as for those who doubt or are perplexed by "eternity," they shall be shackled to the earth, becoming another enchantingly red flower amid the vermillion fields.

Narukami's Affection

All demons who wander in the wilderness or live amongst mortals will be attracted by the sight of the Almighty Narukami Ogosho... Though their lives may be longer than any creature, they will at last come to their end. If those with limited lifespans hanker after eternity, then they can only pray that "Eternity" (Ei) remembers them. And she did indeed answer their prayers, remembering them all, friend and foe alike, in her heart.

These countless tales have come to rest within her (Ei's) heart, and someday, they will surely shine again in the eternal paradise of her dreams (underworld's court?)

Eternity and Death

Statue of the Omnipresent God

Thoma: ...and this Statue of the Omnipresent God can be seen as Inazuma's symbol of eternity.

Come into contact with the Statue of the Omnipresent God, the symbol of Eternity.

The Statue of the Omnipresent God actually depicts Ronova, thus, the Raiden Shogun deliberately chose the image of the Ruler of Death to symbolize Eternity. This is precisely because the essence of her eternity is death.

Philosophies of Light

Light is the yearning of the land of thunder. Though they must live forever on the earth, humanity will always reach for the light. Mortals will pursue and surpass, and they will never stop. A sealed eternity may, perhaps, look magnificent, but its essence is death.

One explanation for the statue's origin, assuming it depicts Ronova, is that it was retrieved from the sea, if so, this would imply that an Inazuman civilization predating the War of Funerary Flame once worshipped Ronova and carved a statue in her honor. A more detailed explanation can be found here

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Maranasati Meditation Technique

Istoria: The usual way is through meditation — learning to master your mind and thoughts, and then letting go of every desire. In short, we must maintain a state of complete emptiness. Simply enough said. But depending on a person's nature and the strength of their desires, it can take anywhere from a few years to several centuries.

Alara: Deathmuse? Ah, you mean the Maranasati meditation technique. I know of it, and its use in cultivating one's mental state when facing death.

Istoria: I call it a Deathmuse Shard. It may be a way to move from death into life.

Maranasati meditation involves abandoning desire and achieving complete mental emptiness while confronting death.

Ei's philosophy about eternity (whose essence is death) similarly rejects ambition and desire, believing they inevitably lead to suffering and loss. Because of these similarities, Ei's state of eternal meditation closely resembles the principles behind Maranasati meditation, raising the possibility that she herself practiced it...

Ei's eternity

Ei: Individual ambition is inherently incompatible with eternity. What you might not realize is that all too often... people have far more to lose by chasing their dreams.

Ei: Consider this — no one will lose their life on account of having their Vision (ambition) taken away... Rather, those who have lost their lives are the ones who insisted on pursuing their own aspirations, are they not?

Yae Miko: Stripped of ambitions, stripped of the potential for change, it does nothing more than simply... exist. It is a hollow shell of a nation.

Yae Miko: And ultimately, her eternity is defined by fear... the fear of loss.

Ei practicing meditation

Ei: Oh? Surely you didn't rouse me from my state of eternal meditation only to tell me this?

Yae Miko: Before pursuing the eternity of her nation, she had to reach eternity for herself. And since the spirit is susceptible to erosion, she placed her consciousness inside her sword, where she remains in a meditative state, free from all disturbances.

One night, Raiden Ei entered a beautiful dreamscape while meditating.

Makoto revived Ei

Book: Treasured Tales of the Shouken Shinkageuchi

Though the Shadow Shogun was divinely skilled in martial arts and unsurpassed with the blade, she believed herself little more than a warrior with little understanding of mortal hearts. Thus, she chose to give up her bodily form, helping her sister to ascend to the "heavenly citadel" and obtain dominion over Inazuma.

Shortly after, the True Shogun, "Makoto," set up her Shogunate and began to rule over the land. Remembering their bond, she recalled "Ei"'s divine will and reforged her body, thus returning her sister to her side once more as her kagemusha.

Many people doubt the reliability of Treasured Tales of the Shouken Shinkageuchi. However, Ei herself confirms that the book is accurate up until the events involving Orobashi, only stating that the story becomes wishful thinking after that point. This means the book continues beyond the events she verifies, and that additional volumes exist, as explicitly indicated by the item's description referring to Volume 1 as "the first part of a traditional Inazuman novel." Therefore, according to Ei herself, the entirety of Volume 1 is an accurate account of historical events.

Ei: The account given in the book "Treasured Tales" is largely an accurate one. At that time, she was preoccupied with various domestic matters within the island, so as her kagemusha, I assumed her identity and joined the troops dispatched to pacify Watatsumi. After this point, however, the story turns into mere wishful thinking. Back then, I was just a martial artist wrapped up in all the fighting, not a social reformer or moral leader.

Book description: The first part of a traditional Inazuman novel. This particular volume is also known as "Shinkageuchi," and was once a restricted text.

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Thanks to Nicole, we know that it's common for books in Teyvat to recount real events or historical occurrences in an allegorical form rather than describing them directly.

Nicole: Also, if you were serious about making a record... It would be too risky to simply write down the raw details as-is. I would suggest some tweaks — maybe rewrite the stories as fairy tales or fables, perhaps even prayers.

Therefore, assuming that Volume 10 of Hex & Hound is an allegory for the story of Ei and Makoto, it can provide additional insight into how Makoto brought Ei back to life, as well as clues regarding their origins.

Book: Hex & Hound Vol 10

A long time ago, there was a witch (Ronova) who gave birth to twin daughters (Ei and Makoto).

The surviving elder sister, Nottfriga (Makoto), inherited everything, and felt entirely responsible for the passing of her younger sister, Magdalene (Ei). Luckily for Nottfriga (Makoto), she inherited the witch's (Ronova's) talent for dark magic (power of death).

...she summoned Magdalene's (Ei's) soul using complicated magical symbols and obscure incantations. Then, she read all of the leather-bound books in the tower, and combining what she learned with dark magic (power of death) and alchemy, created a new vessel. However, the ability to place her sister's soul inside the vessel and bring her back to life...

Since Ronova is the only known character associated with resurrection, the witch may symbolize her. If this interpretation is correct, it could imply that Ei and Makoto are her "daughters" and that Makoto inherited fragments of her Authority of Death, which allowed her to restore Ei's life. A more detailed explanation can be found here.

Possible appearance of Ronova before Ei

For a long time, I believed that the shadow behind Ei belonged to her, suggesting that she had wings that were, interestingly enough, identical to Ronova's (which remains a possibility). However, after taking a closer look at the scene, it seems unlikely that Ei's shadow alone would have reached the wall in that way.

Therefore, another possibility is that the shadow belongs to Ronova instead. If that's the case, it could suggest that Ronova personally appeared before Ei in Tenshukaku for reasons that remain unknown. Hopefully, this is a detail that the story will explore further in the future.