Suggest you look at the oracles of Delphi as an example of an enigmatic text.
There are the Old English Riddles.
You might want to look at Zen Koans such as those by Joshu.
Dogen Kigen of Soto Zen fame can be cryptic.
Some of the Vedas and Upanishads are difficult to understand.
I Ching hexagram descriptions take the cake. But the Classic of the Great Dark is even more extreme in its difficulty.
Alchemical Texts are known for their impenetrable qualities vis a vis the comprehension by the mind.
There are also the Continental Philosophers that many find extremely enigmatic, but there mostly it is a matter of knowing the context of the works within their tradition.
Especially Heideggers Contributions to Philosophy (from Ereignis).
The works of Lacan, despite Zizek’s valiant efforts are some of the most obscure texts I have read.
Hegel’s Phenomenology of Spirit/Ghost/Mind is thought to be the most difficult book of philosophy.
There is the poetry of William Bronk, which can be obscure to say the least.
I find Egyptian Funerary texts to be downright incomprehensible.
Blake’s Four Zoas is not easy to grasp.
But the most difficult text that I know is Finnegan’s Wake.