Undines are a category of elemental beings associated with water, stemming from the alchemical writings of Paracelsus.
Paracelsus (c. 1493 – 24 September 1541), born Theophrastus von Hohenheim (full name Philippus Aureolus Theophrastus Bombastus von Hohenheim), was a Swiss physician, alchemist, lay theologian, and philosopher of the German Renaissance. He is credited as the "father of toxicology".
Paracelsus believed that each of the four classical elements – earth, water, air and fire (and later aether) – is inhabited by different categories of elemental spirits; liminal creatures that share our world: gnomes, undines, sylphs, and salamanders. The earth, water, wind, and fire are a central focus in Sorcery: Contested Realm; and you will also find the creature types across several artworks/cards. A few examples shown below:
From Left to Right
Gneissgnath Gnomes (Vasiliy Ermolaev), Anui Undine (Tony Szczudlo),
Wind Sylph (Elvira Shakirova), Fire Salamander (Melissa Benson)
The word "Undine" derives from the Latin word "unda", meaning "wave", and first appears in Paracelsus' A Book on Nymphs, Sylphs, Pygmies, and Salamanders, and on the Other Spirits, published posthumously in 1566.
Undines are almost always depicted as being female, which is consistent with the ancient Greek idea that water is a female element. Later, writers embellished Paracelsus' undine classification by developing it into a water nymph. A nymph in ancient Greek folklore is a minor female nature deity, generally regarded as personifications of nature.
And thus Erik Oloffson's simple art direction for "Anui Undine" was for Tony to include a large aquatic elemental deity, emerging from a large wave.
Artist Tony Szczudlo is well known for the story-telling behind his artwork. He often includes significant elements in his art design, and current events from his personal life. For this particular piece he reflected on an argument he had with his daughter.
The female image in the illustration is Tony's daughter, and the large crashing wave is a metaphor for his crushed spirit he was feeling at the time while his daughter would not speak with him.