Kriemhild Gretchen is the witch form of Madoka Kaname, and likely the most powerful Witch in existance. She appears in second and fourth timelines of Episode 10; a witch form of Madoka (whether she can be considered Kriemhild is up for debate) also appears in episode 12.
In the second timeline, her formation involves thunderclouds coalescing initially into a giant humanoid and then she seems to form a mountain-like being.
In the fourth timeline, she is shown as a giant creature, actually the size of a mountain, though seen only as a silhouette. Her name was not shown at all during initial broadcast, but was added in the fourth timeline in a webcast version one week later. According to Kyubey, strength of this incarnation of hers is of such a magnitude that she could obliterate the planet within ten days.
Her appearance in both timelines is essentially identical (if one looks closely at her fourth timeline appearance, they will see the head at the top of the shadow between the outstretched arms), though in the second timeline only her head was really shown at all, with the body still forming.
In episode 12, Madoka's all-powerful witch form, who may or may not be considered another form of Kriemhild Gretchen, appears at the end of the universe, born from all the grief Madoka had shouldered from the magical girls she had saved, due to her supremely hopeful wish. She inhabits a comet-sized grief seed and, unlike the second and fourth timelines' versions of Kriemhild, has a fully-shown face (although the left half is either blank or covered in shadow); her head is sun disc-shaped, her open mouth resembles the moon, and her right eye resembles the earth. She is slain by the "goddess" form of Madoka before she can consume the earth.
"Witch of salvation. Her nature is mercy. She absorbs any life on the planet into her newly created heaven--her barrier. The only way to defeat this witch is to make the world free of misfortune. If there's no grief in this world, she will believe this world is already a heaven."
- This Witch may be based on the phenomenon of the Brocken Spectre, an optical illusion sometimes ocurring in mountain ranges which produces dramatically massive, filamentous shadows. This is further consolidated by the fact that mount Brocken is associated with Walpurgisnacht, the inspiration for the name of Walpurgis Night. This could refer to the fact that Madoka, a seemingly small and insignificant girl, produces a disproportionately massive Witch.
- Interestingly, the forms of Walpurgis Night and Kriemhild can be combined to create a sand timer-like shape.