﴿ هُدًۭى وَرَحْمَةًۭ لِّلْمُحْسِنِينَ ﴾

“providing guidance and grace unto the doers of good”

it is a guidance and a mercy read nominative rahmatun; the popular accusative reading rahmatan would make it a circumstantial qualifier referring to āyāt ‘signs’ the operator of which being the implicit demonstrative import of tilka ‘those’ for the virtuous