Lightening - lightning
'To lighten' is like 'to darken' ('to make darker') - only in the opposite direction. It means 'to make lighter', and can be used about either the real world and its visibility, its colours or metaphorically, about emotions, moods etc. Even when it is used as an '-ing participle' ("After the storm, the skies were lightening"), its second syllable is written with an '-e-'.
The flashes that go with thunder have no '-e-' ever - lightning. This is true even where the word is applied to those forms of electrical discharge, like 'sheet' or 'summer lightning', that are accompanied by no sound. (There is no such word, in any form, as
darkning - though Thomas Hardy (1840-1928) used darkling, notably in his poem The Darkling Thrush.)
The word that means 'very fast' is like the electrical discharge - "He ran like lightning to fetch the keys she had left behind"; "Her reactions at tennis were lightning fast."