Case position of an indeclinable relative

Relative Nouns — Ṣilah, ʿĀʾid, Forms and Syntax

«جَاءَ الَّذِي نَجَحَ»: indeclinable relative, nominative position (subject).
«رَأَيْتُ الَّذِي نَجَحَ»: accusative position (object).
«مَرَرْتُ بِالَّذِي نَجَحَ»: genitive position (after preposition).

Its form stays fixed; its syntactic position changes.