Azal poisoned Baha'u'llah with mercury chloride

Jealousy on the part of Azal, intensified by his lust for leadership, grew pejoratively deeper over the course of the Edirne period. As evidence of this, Baha’u’llah apostrophizes ‘you’ who ‘resolved to shed His [Baha’u’llah’s] blood’ (par. 26). This clearly refers to Baha’u’llah’s half-brother and arch-rival, Mīrzā Yaḥyā (‘Azal’), who poisoned Baha’u’llah, ‘probably’ with ‘sublimate of mercury’ (mercury chloride).

Then, failing that, Azal tried to persuade Baha’u’llah’s personal attendant and barber, Ustād Muḥammad-‘Alī Salmānī, to slit Baha’u’llah’s throat.

(Baha’u’llah’s Paradise of Justice: Commentary and Translation, Christopher Buck - Independent Scholar, Pittsburgh, Adib Masumian - Austin)

https://www.researchgate.net/publication/322466992_Baha%27u%27llah%27s_Paradise_of_Justice_Commentary_and_Translation