Everyone has looked at the grimoire cards at least 5 times before and we all have our favorites. My favorite grimoire card is the one about the Relic Aegis. It has a quote from the legendary titan Kabr's last words and explains why there is a pool of light to get cleansed.
These are the last words of Kabr, the Legionless:
I have destroyed myself to do this. They have taken my Ghost. They are in my blood and brain. But now there is hope.
I have made a wound in the Vault. I have pierced it and let in the Light. Bathe in it, and be cleansed. Look to it, and understand:
From my own Light and from the thinking flesh of the Vex I made a shield. The shield is your deliverance. It will break the unbreakable. It will change your fate.
Bind yourself to the shield. Bind yourself to me. And if you abandon your purpose, let the Vault consume you, as it consumed me.
Now it is done. If I speak again, I am not Kabr.
Also the card for Crota's end tells all the guardians who went in and died
Eris (still alive)
Eriana-3
Toland
Agah
Mota
Tarlowe
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Mine would be the one about Crotas Heart its written by Eris and is pretty interesting, I think shes talking to her ghost in this but here it is if you haven't gotten it yet. Eris: Record this. The Heart of Crota. It is her blood that feeds their fury. I thought Omar dead until I heard his screams. I followed them down, to the darkest night of the caverns below. What I saw—I witnessed all we fear—the villainy of the Hive on full display. Among a sea of cocoons, and surrounded by thousands more freshly spawned hordes, the Heart held Omar’s broken body in a vice of bone and pain. She was peeling the Light from his body. How? I can’t imagine, and I have tried. Tendrils of luminance tore away like flesh. With every strand Omar’s scream cut the dark and was met with a chittering chorus from the unborn. I can’t say if they were feeding off the Light itself, or the pain, but my guess is both—somehow, both. The Heart, though I can’t believe she actually has one, seemed to be conducting some nightmare orchestra, nurturing Crota’s children, with the echoes of Agah’s Light. The Hive must end for all they had done, and some day, by my hand or another’s, the Heart will meet with an end fitting of the pain she, herself, has dealt.