The headword, neuter singular of this adjective, is presumably extracted from the quotation.
[1] A tantalizing (late) quotation, from an unknown source and concerning an unidentifiable individual and context. Nor is it self-evident how the 'new-fangled siege-engine' operates. If it is directly connected with what happens on the river banks, it might be a dam or suchlike.
The verb kaqeno/w is a hapax; so is the adverb taxuergw=s (though Xenophon and, especially, Appian have cognates). The passage uses "Thucydidean" shibboleths (e)s, qala/tths), but likewise that tells us little.
"Dariuses" are not otherwise attested with a meaning which makes sense here; the word as transmitted may well be corrupt.