Ligature for the nt digraph

IMHO, this macro nt works nice with both upright and italics fonts in 10-12pt CM, but obviously will need some adjustments using another font or different font sizes:

documentclass[11pt]{article}
makeatletter
newcommand{nt}{%
defstatusem{it}%
ifxf@shapestatusem%
nkern-.105emraisebox{-0.25em}{t}else
nkern-0.27emraisebox{-0.4em}{t}fi}
makeatother
begin{document} 
    audiunt audiunt parem 
    audiunt audiunt
end{document}

As commented, a general solution is impossible, but having an already made general digraph could work well in some cases without adjustments saving the digraph alone in a pdf, and the using the pdf as an image, of course, with a relative size (i.e., with the width in em).

For the italics you cannot make the same kind of joining because the "n" tail is already a broken ligature and you must use it as connect point for best results.

The example above show join the "t" at the left extreme of the horizontal bar, but another reasonable options could be point to the middle of this bar, to simulate and ascending trace to the top of the "t", or from the bottom, without descending the second character in this case:

ntquad  nkern-.16emraisebox{-0.25em}{t}quad nkern-.13em t

For my taste the horizontal bar is too near to the "n" in the second and third version, although the third have the advantage of look more conventional. Anyway, what is better is up to you.

But another possibility is not make the ligature with "n", but with the default eta, that have not right tail:

audiu$eta$kern-.365emraisebox{-0.4em}{t}