Irisviel von Einzbern was not an ordinary human being. She was a homunculus who was created by the Einzbern family for the sole purpose of becoming the lesser vessel to the Holy Grail.
In the Fuyuki Grail Wars, there was an actual lesser vessel (a cup). But it was destroyed before the summoning ritual’s completion. To make sure that this doesn’t happen again, the Einzbern family made an artificial lesser vessel that was capable of defending itself – a homunculus. Her long silver hair, white skin and crimson eyes depict a genetic resemblance to her ancestor Justeaze Lizrich von Einzbern. She formed the core of the Holy Grail when the Holy Grail Wars began.
After meeting with Emiya Kiritsugu, she got a sense of self and experienced human-like emotions which no homunculi ever did before. Gradually, she developed a better understanding and a colorful charisma. This led to her being more wise and mature. She loves Kiritsugu with all her heart and also acts as a proxy master to Saber at times.
What happened to Irisviel in Fate Zero?
As she was a vessel, Irisviel eventually merges with the Holy Grail in Fate:Zero. The Grail which then adopts her personality after her consciousness is transferred to it .
As Irisviel was the Lesser Vessel, she was progressively becoming weaker and weaker as the servants were killed. When five of the servants are dead, the Lesser Grail (lesser vessel) starts to fuse with the Greater Grail i.e. the actual Holy Grail. It was Avalon, which Kiritsugu had given her, that gave her sufficient energy to survive. It also saved her from Kirei Kotomine, when she and Maya were attacked in the forest. However, she returns the Avalon to Kiritsugu before being kidnapped by Kirei.
After knowing the true intentions of Emiya through Irisviel, Kirei chokes her to death. But before this happened, she had already started to merge with the Holy Grail. Her consciousness after her death was also shifted to the vessel.
It was at this moment she realised that the chains of fate which bound her will never be broken. More and more homunculus will be made again and again. This is also the very thing that she wanted to stop. The making of more homunculus and to save her only child, Ilya, from her miserable destiny. However, she failed to do so.
In her visions, Ilya talks about her dream where she was a cup and seven big lumps resided inside her. This pointed towards the same future for her as that of her mother.
Was Irisviel the Holy Grail?
To answer in short, Irisviel was not the Holy Grail. She was only a vessel (also known as Lessel Grail) that was served the purpose of acting as a mediator for the winner when granting a wish.
The Greater Grail had some form of sentient in it and to express it in a tangible manner, it uses Irisviel’s persona. The Grail becomes Irisviel. She is not reborn there but the Grail just took all the aspects that make Irisviel her true self. All of her consciousness is imbibed in the Grail now. This was done in order to talk with the winner of the war, Emiya Kiritsugu. When Kiritsugu kills Irisviel, he is killing the ‘sentient’ of Holy Grail, more like he’s destroying the Grail by rejecting his wish.
The purpose of Irisviel’s life was to sacrifice herself in the end for the sake of the winner of the war and also to bring back the heroic spirits to their designated thrones.
So when someone says that Irisviel is the Holy Grail, they aren’t completely wrong but not right either. Her consciousness was the mediator and her body was the vessel, for the Holy Grail to become an omnipotent wish-granting device, she was needed.
A homunculus is a not some substitute but an essential part of the Holy Grail. Without it, the Grail wouldn’t function and it would be incomplete. What makes Irisviel different is that she was aware of what’s coming and did it unhesitatingly. She wholeheartedly accepted her demise and gracefully realised her true purpose.
She was content in her last moments, as she knew that she had made some great memories with her husband and their daughter, Illyasviel.