Form the possessive singular of nouns with 's. Follow this rule whatever the final consonant. Thus write, Charles's friend Burns's poems the witch's malice
Possessive singular ends with 's, excerpt for ancient possessive proper names in -es and -is, Jesus', and "such forms as"(*?) righteousness' and consciousness'. Another is "Achilles' heel" (for the sake of equation let's use R' for Achilles'(/possessor noun) and D for heel(/possessed noun)). Typically "R' D" is replaced by "D of R" i.e. the heel of Achilles, for the sake of consciousness, etc.
*Presumably a noun, not necessarily abstract though perhaps, ending with a "s" sound.