百毒不侵只是曾经
[băi dú bù qīn zhĭ shì céng jīng]
Literally meaning 'Invulnerable to all poisons but only in the past', it suggests a transformation from having once been unassailable or resilient (metaphorically 'immune to poisons') to no longer possessing that quality. The user might have undergone experiences that have altered their once untouchable stance or demeanor.