undreamed vs undreamt

undreamed and undreamt both are adjectives.

Word	Noun	Adjective	Verb	Adverb
undreamed	No	Yes	No	No
undreamt	No	Yes	No	No

As adjectives, undreamed and undreamt are synonyms defined as:

Other synonyms of undreamed include undreamed of, undreamt of, unimagined.

undreamed (adjective)	undreamt (adjective)
not imagined even in a dream	not imagined even in a dream