This article contains spoilers
Kriemhild Gretchen is one of the witches in Puella Magi Madoka Magica. First appeared in episode 10, and probably, or most likely the strongest witch ever, reaching universal level threat.
This witch is what Madoka Kaname had become (or in certain timelines, would have become) if her Soul Gem is not purified and exerted herself too much into defeating her opponents.
According to the official webpage, her witch designation is categorized as Witch of salvation with her nature as "Mercy".
She is also labelled as the worst witch ever. Where she will turn into one once Walpurgisnacht is defeated. She is seen transformed into one on at ep.10, where Homura witnessed the change, shocking her and made her realize that she had been tricked into becoming a Puella Magi.
According to Kyuubey, with her transformation, added with Homura's numerous time travel, all living things on Earth will be destroyed in 10 days if left alone. While it is theorized that she can be destroyed using WMD - considering her designation being the witch of salvation, There is no saying on whether such action taken will guarantee its death.
While her wish might be different in each timeline, it is assumed that her wish to save someone or something might be consistent at each timeline. While her wish at the first timeline was to save a cat from an accident, other timelines might be different.
At the end of episode 12. Her witch form has evolved up to the point of being at the massive, gigantic, universe size that was born from Madoka upon absorbing all the Soul Gem's grief of fellow Puellae Magi, which Kyuubey claimed that this curse is born from the result of rewriting the laws of the universe, which is enough to destroy the universe itself. However, it is cut short considering Madoka's wish is to have the witches completely disappear and rid off from the universe.
According to the official guidebook, it is confirmed that the Kriemhild Gretchen from the final episode is born from all the despair accumulated from Puellae Magis, and a form representing no one, Witch of Despair.
Her form in episode 10 which is shaped to look like a gigantic mountain
According to witch animator and designer InuCurry, Walpurgis Night's and Kriemhild Gretchen's silhouettes are supposed to make a pair. The Puella Magi Production Note further reveals that Kriemhild Gretchen is meant to look like the lower half of a sand timer, while Walpurgis Night looks like the upper half. Interestingly, Homura's shield has also been described as a sand timer. This is assumed that she is shaped that way as the result of Homura's time travels which added to Madoka's karma.
During the secomd timeline her face is briefly shown, but it was considered incomplete as she was still transforming. Her final outlook remained unknown.
Episode 12 version. Her face is shaped to be somewhat like a black sun, which is gigantic enough to cover the Earth.
When it was first broadcasted, the witch was not named, and it was assumed to be unknown. The broadcast at NicoNico later confirmed her name.
Kriemhild and Gretchen are actually 2 different names combined into one. Which either one of them are first names.
A German female name
Also known as Gudrun, Kriemhild is a figure from Norse Mythology. She is an important character in the epic poem Nibelungenlied. Note also that Kriemhild comes from the German words for "mask" and "battle". A character that is portrayed with an extreme personality of mercy and revenge.
A German female name
In Goethe's Faust, Gretchen, also known as Margarete, was the name of Faust's lover. She dies at the end of Part 1 of Faust, but returns at the end of Part 2 as a Pentinent in heaven. She is allowed to lead Faust's soul into heaven's higher spheres.
Kriemhild Gretchen is the ONLY witch in the anime series to not have any Familiars. But if one were to look at her designation, she might not need one after all. As she was just born before Homura rewind her time, we might not even get to see whether she actually had one or not.
Familiars created from witches are usually created with the purpose of obtaining something that the witches truly wanted. Considering Kriemhild Gretchen is created to bring salvation, and Madoka herself always wanted to create something by her own. So even if she turned into a witch, she might have no need in actually having familiars of her own.
She is always seen manifested at the reality, so she does not have her own border.
Due to her size and design, she is sometimes called Madoka Mountain, and is still used today.
Artists sometimes draw fanarts depicting whatever Madoka was thinking after she turned into a witch remains unknown due to the contents of the wish being different on each timeline.
She is also portrayed by some artists to be a witch with a cheat code (invincibility), and it is because of that, she is sometimes seen been made fun of. She is also sometimes seen as a rival to Elsamaria due to the fact both of them are covered off and have their true forms remained visually unknown.
There are also personification artworks of Kriemheid Gretchen. The personification of the witch is categorized under Dark Madoka.
Since Goddess Madoka and Kriemhild Gretchen's action is to bring salvation to all, creating a new world. While executions might be different, they have the same motives, hence they were sometimes seen paired and drawn together.