Last time, under a promise to keep it secret from K, I sent a sloppy piece of writing titled 'An Aftertale of Sayū', and this one is the sequel to that aftertale.
Literal
K-san [of-の] secret-to-having [quotative-と] [possessive-の] promise [possessive-の] basis [locative-に] last-time 'Sayū' aftertale being [attributive-なる] humble-writing [object-を] sent-[polite past-ましたが], this-time [topic-は] that [possessive-の] aftertale is.
A densely self-referential blog-style intro. The expression ~のもとに ('under X, on the basis of X') sets up the conditions. 駄文 is a humble self-deprecation for one's own writing, literally 'poor/useless text'. The classical attributive copula なる (ancient predecessor of な/だ) is preserved in quoted titles and literary phrasing: '『左右』後日談なる駄文' = 'a poor piece (that is) the Sayū aftertale'. The loop of 後日談 ('aftertale') followed by 'an aftertale to the aftertale' is self-deprecating humor about padding.